venv

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/conftest.py

@@ -0,0 +1,7 @@
+import pytest
+
+
+@pytest.fixture(params=[True, False])
+def sort(request):
+    """Boolean sort keyword for concat and DataFrame.append."""
+    return request.param

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_append.py

@@ -0,0 +1,389 @@
+import datetime as dt
+from itertools import combinations
+
+import dateutil
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    Index,
+    Series,
+    Timestamp,
+    concat,
+    isna,
+)
+import pandas._testing as tm
+
+
+class TestAppend:
+    def test_append(self, sort, float_frame):
+        mixed_frame = float_frame.copy()
+        mixed_frame["foo"] = "bar"
+
+        begin_index = float_frame.index[:5]
+        end_index = float_frame.index[5:]
+
+        begin_frame = float_frame.reindex(begin_index)
+        end_frame = float_frame.reindex(end_index)
+
+        appended = begin_frame._append(end_frame)
+        tm.assert_almost_equal(appended["A"], float_frame["A"])
+
+        del end_frame["A"]
+        partial_appended = begin_frame._append(end_frame, sort=sort)
+        assert "A" in partial_appended
+
+        partial_appended = end_frame._append(begin_frame, sort=sort)
+        assert "A" in partial_appended
+
+        # mixed type handling
+        appended = mixed_frame[:5]._append(mixed_frame[5:])
+        tm.assert_frame_equal(appended, mixed_frame)
+
+        # what to test here
+        mixed_appended = mixed_frame[:5]._append(float_frame[5:], sort=sort)
+        mixed_appended2 = float_frame[:5]._append(mixed_frame[5:], sort=sort)
+
+        # all equal except 'foo' column
+        tm.assert_frame_equal(
+            mixed_appended.reindex(columns=["A", "B", "C", "D"]),
+            mixed_appended2.reindex(columns=["A", "B", "C", "D"]),
+        )
+
+    def test_append_empty(self, float_frame):
+        empty = DataFrame()
+
+        appended = float_frame._append(empty)
+        tm.assert_frame_equal(float_frame, appended)
+        assert appended is not float_frame
+
+        appended = empty._append(float_frame)
+        tm.assert_frame_equal(float_frame, appended)
+        assert appended is not float_frame
+
+    def test_append_overlap_raises(self, float_frame):
+        msg = "Indexes have overlapping values"
+        with pytest.raises(ValueError, match=msg):
+            float_frame._append(float_frame, verify_integrity=True)
+
+    def test_append_new_columns(self):
+        # see gh-6129: new columns
+        df = DataFrame({"a": {"x": 1, "y": 2}, "b": {"x": 3, "y": 4}})
+        row = Series([5, 6, 7], index=["a", "b", "c"], name="z")
+        expected = DataFrame(
+            {
+                "a": {"x": 1, "y": 2, "z": 5},
+                "b": {"x": 3, "y": 4, "z": 6},
+                "c": {"z": 7},
+            }
+        )
+        result = df._append(row)
+        tm.assert_frame_equal(result, expected)
+
+    def test_append_length0_frame(self, sort):
+        df = DataFrame(columns=["A", "B", "C"])
+        df3 = DataFrame(index=[0, 1], columns=["A", "B"])
+        df5 = df._append(df3, sort=sort)
+
+        expected = DataFrame(index=[0, 1], columns=["A", "B", "C"])
+        tm.assert_frame_equal(df5, expected)
+
+    def test_append_records(self):
+        arr1 = np.zeros((2,), dtype=("i4,f4,S10"))
+        arr1[:] = [(1, 2.0, "Hello"), (2, 3.0, "World")]
+
+        arr2 = np.zeros((3,), dtype=("i4,f4,S10"))
+        arr2[:] = [(3, 4.0, "foo"), (5, 6.0, "bar"), (7.0, 8.0, "baz")]
+
+        df1 = DataFrame(arr1)
+        df2 = DataFrame(arr2)
+
+        result = df1._append(df2, ignore_index=True)
+        expected = DataFrame(np.concatenate((arr1, arr2)))
+        tm.assert_frame_equal(result, expected)
+
+    # rewrite sort fixture, since we also want to test default of None
+    def test_append_sorts(self, sort):
+        df1 = DataFrame({"a": [1, 2], "b": [1, 2]}, columns=["b", "a"])
+        df2 = DataFrame({"a": [1, 2], "c": [3, 4]}, index=[2, 3])
+
+        result = df1._append(df2, sort=sort)
+
+        # for None / True
+        expected = DataFrame(
+            {"b": [1, 2, None, None], "a": [1, 2, 1, 2], "c": [None, None, 3, 4]},
+            columns=["a", "b", "c"],
+        )
+        if sort is False:
+            expected = expected[["b", "a", "c"]]
+        tm.assert_frame_equal(result, expected)
+
+    def test_append_different_columns(self, sort):
+        df = DataFrame(
+            {
+                "bools": np.random.default_rng(2).standard_normal(10) > 0,
+                "ints": np.random.default_rng(2).integers(0, 10, 10),
+                "floats": np.random.default_rng(2).standard_normal(10),
+                "strings": ["foo", "bar"] * 5,
+            }
+        )
+
+        a = df[:5].loc[:, ["bools", "ints", "floats"]]
+        b = df[5:].loc[:, ["strings", "ints", "floats"]]
+
+        appended = a._append(b, sort=sort)
+        assert isna(appended["strings"][0:4]).all()
+        assert isna(appended["bools"][5:]).all()
+
+    def test_append_many(self, sort, float_frame):
+        chunks = [
+            float_frame[:5],
+            float_frame[5:10],
+            float_frame[10:15],
+            float_frame[15:],
+        ]
+
+        result = chunks[0]._append(chunks[1:])
+        tm.assert_frame_equal(result, float_frame)
+
+        chunks[-1] = chunks[-1].copy()
+        chunks[-1]["foo"] = "bar"
+        result = chunks[0]._append(chunks[1:], sort=sort)
+        tm.assert_frame_equal(result.loc[:, float_frame.columns], float_frame)
+        assert (result["foo"][15:] == "bar").all()
+        assert result["foo"][:15].isna().all()
+
+    def test_append_preserve_index_name(self):
+        # #980
+        df1 = DataFrame(columns=["A", "B", "C"])
+        df1 = df1.set_index(["A"])
+        df2 = DataFrame(data=[[1, 4, 7], [2, 5, 8], [3, 6, 9]], columns=["A", "B", "C"])
+        df2 = df2.set_index(["A"])
+
+        msg = "The behavior of array concatenation with empty entries is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = df1._append(df2)
+        assert result.index.name == "A"
+
+    indexes_can_append = [
+        pd.RangeIndex(3),
+        Index([4, 5, 6]),
+        Index([4.5, 5.5, 6.5]),
+        Index(list("abc")),
+        pd.CategoricalIndex("A B C".split()),
+        pd.CategoricalIndex("D E F".split(), ordered=True),
+        pd.IntervalIndex.from_breaks([7, 8, 9, 10]),
+        pd.DatetimeIndex(
+            [
+                dt.datetime(2013, 1, 3, 0, 0),
+                dt.datetime(2013, 1, 3, 6, 10),
+                dt.datetime(2013, 1, 3, 7, 12),
+            ]
+        ),
+        pd.MultiIndex.from_arrays(["A B C".split(), "D E F".split()]),
+    ]
+
+    @pytest.mark.parametrize(
+        "index", indexes_can_append, ids=lambda x: type(x).__name__
+    )
+    def test_append_same_columns_type(self, index):
+        # GH18359
+
+        # df wider than ser
+        df = DataFrame([[1, 2, 3], [4, 5, 6]], columns=index)
+        ser_index = index[:2]
+        ser = Series([7, 8], index=ser_index, name=2)
+        result = df._append(ser)
+        expected = DataFrame(
+            [[1, 2, 3.0], [4, 5, 6], [7, 8, np.nan]], index=[0, 1, 2], columns=index
+        )
+        # integer dtype is preserved for columns present in ser.index
+        assert expected.dtypes.iloc[0].kind == "i"
+        assert expected.dtypes.iloc[1].kind == "i"
+
+        tm.assert_frame_equal(result, expected)
+
+        # ser wider than df
+        ser_index = index
+        index = index[:2]
+        df = DataFrame([[1, 2], [4, 5]], columns=index)
+        ser = Series([7, 8, 9], index=ser_index, name=2)
+        result = df._append(ser)
+        expected = DataFrame(
+            [[1, 2, np.nan], [4, 5, np.nan], [7, 8, 9]],
+            index=[0, 1, 2],
+            columns=ser_index,
+        )
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "df_columns, series_index",
+        combinations(indexes_can_append, r=2),
+        ids=lambda x: type(x).__name__,
+    )
+    def test_append_different_columns_types(self, df_columns, series_index):
+        # GH18359
+        # See also test 'test_append_different_columns_types_raises' below
+        # for errors raised when appending
+
+        df = DataFrame([[1, 2, 3], [4, 5, 6]], columns=df_columns)
+        ser = Series([7, 8, 9], index=series_index, name=2)
+
+        result = df._append(ser)
+        idx_diff = ser.index.difference(df_columns)
+        combined_columns = Index(df_columns.tolist()).append(idx_diff)
+        expected = DataFrame(
+            [
+                [1.0, 2.0, 3.0, np.nan, np.nan, np.nan],
+                [4, 5, 6, np.nan, np.nan, np.nan],
+                [np.nan, np.nan, np.nan, 7, 8, 9],
+            ],
+            index=[0, 1, 2],
+            columns=combined_columns,
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_append_dtype_coerce(self, sort):
+        # GH 4993
+        # appending with datetime will incorrectly convert datetime64
+
+        df1 = DataFrame(
+            index=[1, 2],
+            data=[dt.datetime(2013, 1, 1, 0, 0), dt.datetime(2013, 1, 2, 0, 0)],
+            columns=["start_time"],
+        )
+        df2 = DataFrame(
+            index=[4, 5],
+            data=[
+                [dt.datetime(2013, 1, 3, 0, 0), dt.datetime(2013, 1, 3, 6, 10)],
+                [dt.datetime(2013, 1, 4, 0, 0), dt.datetime(2013, 1, 4, 7, 10)],
+            ],
+            columns=["start_time", "end_time"],
+        )
+
+        expected = concat(
+            [
+                Series(
+                    [
+                        pd.NaT,
+                        pd.NaT,
+                        dt.datetime(2013, 1, 3, 6, 10),
+                        dt.datetime(2013, 1, 4, 7, 10),
+                    ],
+                    name="end_time",
+                ),
+                Series(
+                    [
+                        dt.datetime(2013, 1, 1, 0, 0),
+                        dt.datetime(2013, 1, 2, 0, 0),
+                        dt.datetime(2013, 1, 3, 0, 0),
+                        dt.datetime(2013, 1, 4, 0, 0),
+                    ],
+                    name="start_time",
+                ),
+            ],
+            axis=1,
+            sort=sort,
+        )
+        result = df1._append(df2, ignore_index=True, sort=sort)
+        if sort:
+            expected = expected[["end_time", "start_time"]]
+        else:
+            expected = expected[["start_time", "end_time"]]
+
+        tm.assert_frame_equal(result, expected)
+
+    def test_append_missing_column_proper_upcast(self, sort):
+        df1 = DataFrame({"A": np.array([1, 2, 3, 4], dtype="i8")})
+        df2 = DataFrame({"B": np.array([True, False, True, False], dtype=bool)})
+
+        appended = df1._append(df2, ignore_index=True, sort=sort)
+        assert appended["A"].dtype == "f8"
+        assert appended["B"].dtype == "O"
+
+    def test_append_empty_frame_to_series_with_dateutil_tz(self):
+        # GH 23682
+        date = Timestamp("2018-10-24 07:30:00", tz=dateutil.tz.tzutc())
+        ser = Series({"a": 1.0, "b": 2.0, "date": date})
+        df = DataFrame(columns=["c", "d"])
+        result_a = df._append(ser, ignore_index=True)
+        expected = DataFrame(
+            [[np.nan, np.nan, 1.0, 2.0, date]], columns=["c", "d", "a", "b", "date"]
+        )
+        # These columns get cast to object after append
+        expected["c"] = expected["c"].astype(object)
+        expected["d"] = expected["d"].astype(object)
+        tm.assert_frame_equal(result_a, expected)
+
+        expected = DataFrame(
+            [[np.nan, np.nan, 1.0, 2.0, date]] * 2, columns=["c", "d", "a", "b", "date"]
+        )
+        expected["c"] = expected["c"].astype(object)
+        expected["d"] = expected["d"].astype(object)
+        result_b = result_a._append(ser, ignore_index=True)
+        tm.assert_frame_equal(result_b, expected)
+
+        result = df._append([ser, ser], ignore_index=True)
+        tm.assert_frame_equal(result, expected)
+
+    def test_append_empty_tz_frame_with_datetime64ns(self, using_array_manager):
+        # https://github.com/pandas-dev/pandas/issues/35460
+        df = DataFrame(columns=["a"]).astype("datetime64[ns, UTC]")
+
+        # pd.NaT gets inferred as tz-naive, so append result is tz-naive
+        result = df._append({"a": pd.NaT}, ignore_index=True)
+        if using_array_manager:
+            expected = DataFrame({"a": [pd.NaT]}, dtype=object)
+        else:
+            expected = DataFrame({"a": [np.nan]}, dtype=object)
+        tm.assert_frame_equal(result, expected)
+
+        # also test with typed value to append
+        df = DataFrame(columns=["a"]).astype("datetime64[ns, UTC]")
+        other = Series({"a": pd.NaT}, dtype="datetime64[ns]")
+        result = df._append(other, ignore_index=True)
+        tm.assert_frame_equal(result, expected)
+
+        # mismatched tz
+        other = Series({"a": pd.NaT}, dtype="datetime64[ns, US/Pacific]")
+        result = df._append(other, ignore_index=True)
+        expected = DataFrame({"a": [pd.NaT]}).astype(object)
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "dtype_str", ["datetime64[ns, UTC]", "datetime64[ns]", "Int64", "int64"]
+    )
+    @pytest.mark.parametrize("val", [1, "NaT"])
+    def test_append_empty_frame_with_timedelta64ns_nat(
+        self, dtype_str, val, using_array_manager
+    ):
+        # https://github.com/pandas-dev/pandas/issues/35460
+        df = DataFrame(columns=["a"]).astype(dtype_str)
+
+        other = DataFrame({"a": [np.timedelta64(val, "ns")]})
+        result = df._append(other, ignore_index=True)
+
+        expected = other.astype(object)
+        if isinstance(val, str) and dtype_str != "int64" and not using_array_manager:
+            # TODO: expected used to be `other.astype(object)` which is a more
+            #  reasonable result.  This was changed when tightening
+            #  assert_frame_equal's treatment of mismatched NAs to match the
+            #  existing behavior.
+            expected = DataFrame({"a": [np.nan]}, dtype=object)
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "dtype_str", ["datetime64[ns, UTC]", "datetime64[ns]", "Int64", "int64"]
+    )
+    @pytest.mark.parametrize("val", [1, "NaT"])
+    def test_append_frame_with_timedelta64ns_nat(self, dtype_str, val):
+        # https://github.com/pandas-dev/pandas/issues/35460
+        df = DataFrame({"a": pd.array([1], dtype=dtype_str)})
+
+        other = DataFrame({"a": [np.timedelta64(val, "ns")]})
+        result = df._append(other, ignore_index=True)
+
+        expected = DataFrame({"a": [df.iloc[0, 0], other.iloc[0, 0]]}, dtype=object)
+        tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_append_common.py

@@ -0,0 +1,753 @@
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import (
+    Categorical,
+    DataFrame,
+    Index,
+    Series,
+)
+import pandas._testing as tm
+
+
+@pytest.fixture(
+    params=list(
+        {
+            "bool": [True, False, True],
+            "int64": [1, 2, 3],
+            "float64": [1.1, np.nan, 3.3],
+            "category": Categorical(["X", "Y", "Z"]),
+            "object": ["a", "b", "c"],
+            "datetime64[ns]": [
+                pd.Timestamp("2011-01-01"),
+                pd.Timestamp("2011-01-02"),
+                pd.Timestamp("2011-01-03"),
+            ],
+            "datetime64[ns, US/Eastern]": [
+                pd.Timestamp("2011-01-01", tz="US/Eastern"),
+                pd.Timestamp("2011-01-02", tz="US/Eastern"),
+                pd.Timestamp("2011-01-03", tz="US/Eastern"),
+            ],
+            "timedelta64[ns]": [
+                pd.Timedelta("1 days"),
+                pd.Timedelta("2 days"),
+                pd.Timedelta("3 days"),
+            ],
+            "period[M]": [
+                pd.Period("2011-01", freq="M"),
+                pd.Period("2011-02", freq="M"),
+                pd.Period("2011-03", freq="M"),
+            ],
+        }.items()
+    )
+)
+def item(request):
+    key, data = request.param
+    return key, data
+
+
+@pytest.fixture
+def item2(item):
+    return item
+
+
+class TestConcatAppendCommon:
+    """
+    Test common dtype coercion rules between concat and append.
+    """
+
+    def test_dtypes(self, item, index_or_series, using_infer_string):
+        # to confirm test case covers intended dtypes
+        typ, vals = item
+        obj = index_or_series(vals)
+        if typ == "object" and using_infer_string:
+            typ = "string"
+        if isinstance(obj, Index):
+            assert obj.dtype == typ
+        elif isinstance(obj, Series):
+            if typ.startswith("period"):
+                assert obj.dtype == "Period[M]"
+            else:
+                assert obj.dtype == typ
+
+    def test_concatlike_same_dtypes(self, item):
+        # GH 13660
+        typ1, vals1 = item
+
+        vals2 = vals1
+        vals3 = vals1
+
+        if typ1 == "category":
+            exp_data = Categorical(list(vals1) + list(vals2))
+            exp_data3 = Categorical(list(vals1) + list(vals2) + list(vals3))
+        else:
+            exp_data = vals1 + vals2
+            exp_data3 = vals1 + vals2 + vals3
+
+        # ----- Index ----- #
+
+        # index.append
+        res = Index(vals1).append(Index(vals2))
+        exp = Index(exp_data)
+        tm.assert_index_equal(res, exp)
+
+        # 3 elements
+        res = Index(vals1).append([Index(vals2), Index(vals3)])
+        exp = Index(exp_data3)
+        tm.assert_index_equal(res, exp)
+
+        # index.append name mismatch
+        i1 = Index(vals1, name="x")
+        i2 = Index(vals2, name="y")
+        res = i1.append(i2)
+        exp = Index(exp_data)
+        tm.assert_index_equal(res, exp)
+
+        # index.append name match
+        i1 = Index(vals1, name="x")
+        i2 = Index(vals2, name="x")
+        res = i1.append(i2)
+        exp = Index(exp_data, name="x")
+        tm.assert_index_equal(res, exp)
+
+        # cannot append non-index
+        with pytest.raises(TypeError, match="all inputs must be Index"):
+            Index(vals1).append(vals2)
+
+        with pytest.raises(TypeError, match="all inputs must be Index"):
+            Index(vals1).append([Index(vals2), vals3])
+
+        # ----- Series ----- #
+
+        # series.append
+        res = Series(vals1)._append(Series(vals2), ignore_index=True)
+        exp = Series(exp_data)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        # concat
+        res = pd.concat([Series(vals1), Series(vals2)], ignore_index=True)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        # 3 elements
+        res = Series(vals1)._append([Series(vals2), Series(vals3)], ignore_index=True)
+        exp = Series(exp_data3)
+        tm.assert_series_equal(res, exp)
+
+        res = pd.concat(
+            [Series(vals1), Series(vals2), Series(vals3)],
+            ignore_index=True,
+        )
+        tm.assert_series_equal(res, exp)
+
+        # name mismatch
+        s1 = Series(vals1, name="x")
+        s2 = Series(vals2, name="y")
+        res = s1._append(s2, ignore_index=True)
+        exp = Series(exp_data)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        res = pd.concat([s1, s2], ignore_index=True)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        # name match
+        s1 = Series(vals1, name="x")
+        s2 = Series(vals2, name="x")
+        res = s1._append(s2, ignore_index=True)
+        exp = Series(exp_data, name="x")
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        res = pd.concat([s1, s2], ignore_index=True)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        # cannot append non-index
+        msg = (
+            r"cannot concatenate object of type '.+'; "
+            "only Series and DataFrame objs are valid"
+        )
+        with pytest.raises(TypeError, match=msg):
+            Series(vals1)._append(vals2)
+
+        with pytest.raises(TypeError, match=msg):
+            Series(vals1)._append([Series(vals2), vals3])
+
+        with pytest.raises(TypeError, match=msg):
+            pd.concat([Series(vals1), vals2])
+
+        with pytest.raises(TypeError, match=msg):
+            pd.concat([Series(vals1), Series(vals2), vals3])
+
+    def test_concatlike_dtypes_coercion(self, item, item2, request):
+        # GH 13660
+        typ1, vals1 = item
+        typ2, vals2 = item2
+
+        vals3 = vals2
+
+        # basically infer
+        exp_index_dtype = None
+        exp_series_dtype = None
+
+        if typ1 == typ2:
+            pytest.skip("same dtype is tested in test_concatlike_same_dtypes")
+        elif typ1 == "category" or typ2 == "category":
+            pytest.skip("categorical type tested elsewhere")
+
+        # specify expected dtype
+        if typ1 == "bool" and typ2 in ("int64", "float64"):
+            # series coerces to numeric based on numpy rule
+            # index doesn't because bool is object dtype
+            exp_series_dtype = typ2
+            mark = pytest.mark.xfail(reason="GH#39187 casting to object")
+            request.applymarker(mark)
+        elif typ2 == "bool" and typ1 in ("int64", "float64"):
+            exp_series_dtype = typ1
+            mark = pytest.mark.xfail(reason="GH#39187 casting to object")
+            request.applymarker(mark)
+        elif typ1 in {"datetime64[ns, US/Eastern]", "timedelta64[ns]"} or typ2 in {
+            "datetime64[ns, US/Eastern]",
+            "timedelta64[ns]",
+        }:
+            exp_index_dtype = object
+            exp_series_dtype = object
+
+        exp_data = vals1 + vals2
+        exp_data3 = vals1 + vals2 + vals3
+
+        # ----- Index ----- #
+
+        # index.append
+        # GH#39817
+        res = Index(vals1).append(Index(vals2))
+        exp = Index(exp_data, dtype=exp_index_dtype)
+        tm.assert_index_equal(res, exp)
+
+        # 3 elements
+        res = Index(vals1).append([Index(vals2), Index(vals3)])
+        exp = Index(exp_data3, dtype=exp_index_dtype)
+        tm.assert_index_equal(res, exp)
+
+        # ----- Series ----- #
+
+        # series._append
+        # GH#39817
+        res = Series(vals1)._append(Series(vals2), ignore_index=True)
+        exp = Series(exp_data, dtype=exp_series_dtype)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        # concat
+        # GH#39817
+        res = pd.concat([Series(vals1), Series(vals2)], ignore_index=True)
+        tm.assert_series_equal(res, exp, check_index_type=True)
+
+        # 3 elements
+        # GH#39817
+        res = Series(vals1)._append([Series(vals2), Series(vals3)], ignore_index=True)
+        exp = Series(exp_data3, dtype=exp_series_dtype)
+        tm.assert_series_equal(res, exp)
+
+        # GH#39817
+        res = pd.concat(
+            [Series(vals1), Series(vals2), Series(vals3)],
+            ignore_index=True,
+        )
+        tm.assert_series_equal(res, exp)
+
+    def test_concatlike_common_coerce_to_pandas_object(self):
+        # GH 13626
+        # result must be Timestamp/Timedelta, not datetime.datetime/timedelta
+        dti = pd.DatetimeIndex(["2011-01-01", "2011-01-02"])
+        tdi = pd.TimedeltaIndex(["1 days", "2 days"])
+
+        exp = Index(
+            [
+                pd.Timestamp("2011-01-01"),
+                pd.Timestamp("2011-01-02"),
+                pd.Timedelta("1 days"),
+                pd.Timedelta("2 days"),
+            ]
+        )
+
+        res = dti.append(tdi)
+        tm.assert_index_equal(res, exp)
+        assert isinstance(res[0], pd.Timestamp)
+        assert isinstance(res[-1], pd.Timedelta)
+
+        dts = Series(dti)
+        tds = Series(tdi)
+        res = dts._append(tds)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+        assert isinstance(res.iloc[0], pd.Timestamp)
+        assert isinstance(res.iloc[-1], pd.Timedelta)
+
+        res = pd.concat([dts, tds])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+        assert isinstance(res.iloc[0], pd.Timestamp)
+        assert isinstance(res.iloc[-1], pd.Timedelta)
+
+    def test_concatlike_datetimetz(self, tz_aware_fixture):
+        tz = tz_aware_fixture
+        # GH 7795
+        dti1 = pd.DatetimeIndex(["2011-01-01", "2011-01-02"], tz=tz)
+        dti2 = pd.DatetimeIndex(["2012-01-01", "2012-01-02"], tz=tz)
+
+        exp = pd.DatetimeIndex(
+            ["2011-01-01", "2011-01-02", "2012-01-01", "2012-01-02"], tz=tz
+        )
+
+        res = dti1.append(dti2)
+        tm.assert_index_equal(res, exp)
+
+        dts1 = Series(dti1)
+        dts2 = Series(dti2)
+        res = dts1._append(dts2)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([dts1, dts2])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+    @pytest.mark.parametrize("tz", ["UTC", "US/Eastern", "Asia/Tokyo", "EST5EDT"])
+    def test_concatlike_datetimetz_short(self, tz):
+        # GH#7795
+        ix1 = pd.date_range(start="2014-07-15", end="2014-07-17", freq="D", tz=tz)
+        ix2 = pd.DatetimeIndex(["2014-07-11", "2014-07-21"], tz=tz)
+        df1 = DataFrame(0, index=ix1, columns=["A", "B"])
+        df2 = DataFrame(0, index=ix2, columns=["A", "B"])
+
+        exp_idx = pd.DatetimeIndex(
+            ["2014-07-15", "2014-07-16", "2014-07-17", "2014-07-11", "2014-07-21"],
+            tz=tz,
+        ).as_unit("ns")
+        exp = DataFrame(0, index=exp_idx, columns=["A", "B"])
+
+        tm.assert_frame_equal(df1._append(df2), exp)
+        tm.assert_frame_equal(pd.concat([df1, df2]), exp)
+
+    def test_concatlike_datetimetz_to_object(self, tz_aware_fixture):
+        tz = tz_aware_fixture
+        # GH 13660
+
+        # different tz coerces to object
+        dti1 = pd.DatetimeIndex(["2011-01-01", "2011-01-02"], tz=tz)
+        dti2 = pd.DatetimeIndex(["2012-01-01", "2012-01-02"])
+
+        exp = Index(
+            [
+                pd.Timestamp("2011-01-01", tz=tz),
+                pd.Timestamp("2011-01-02", tz=tz),
+                pd.Timestamp("2012-01-01"),
+                pd.Timestamp("2012-01-02"),
+            ],
+            dtype=object,
+        )
+
+        res = dti1.append(dti2)
+        tm.assert_index_equal(res, exp)
+
+        dts1 = Series(dti1)
+        dts2 = Series(dti2)
+        res = dts1._append(dts2)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([dts1, dts2])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        # different tz
+        dti3 = pd.DatetimeIndex(["2012-01-01", "2012-01-02"], tz="US/Pacific")
+
+        exp = Index(
+            [
+                pd.Timestamp("2011-01-01", tz=tz),
+                pd.Timestamp("2011-01-02", tz=tz),
+                pd.Timestamp("2012-01-01", tz="US/Pacific"),
+                pd.Timestamp("2012-01-02", tz="US/Pacific"),
+            ],
+            dtype=object,
+        )
+
+        res = dti1.append(dti3)
+        tm.assert_index_equal(res, exp)
+
+        dts1 = Series(dti1)
+        dts3 = Series(dti3)
+        res = dts1._append(dts3)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([dts1, dts3])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+    def test_concatlike_common_period(self):
+        # GH 13660
+        pi1 = pd.PeriodIndex(["2011-01", "2011-02"], freq="M")
+        pi2 = pd.PeriodIndex(["2012-01", "2012-02"], freq="M")
+
+        exp = pd.PeriodIndex(["2011-01", "2011-02", "2012-01", "2012-02"], freq="M")
+
+        res = pi1.append(pi2)
+        tm.assert_index_equal(res, exp)
+
+        ps1 = Series(pi1)
+        ps2 = Series(pi2)
+        res = ps1._append(ps2)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([ps1, ps2])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+    def test_concatlike_common_period_diff_freq_to_object(self):
+        # GH 13221
+        pi1 = pd.PeriodIndex(["2011-01", "2011-02"], freq="M")
+        pi2 = pd.PeriodIndex(["2012-01-01", "2012-02-01"], freq="D")
+
+        exp = Index(
+            [
+                pd.Period("2011-01", freq="M"),
+                pd.Period("2011-02", freq="M"),
+                pd.Period("2012-01-01", freq="D"),
+                pd.Period("2012-02-01", freq="D"),
+            ],
+            dtype=object,
+        )
+
+        res = pi1.append(pi2)
+        tm.assert_index_equal(res, exp)
+
+        ps1 = Series(pi1)
+        ps2 = Series(pi2)
+        res = ps1._append(ps2)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([ps1, ps2])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+    def test_concatlike_common_period_mixed_dt_to_object(self):
+        # GH 13221
+        # different datetimelike
+        pi1 = pd.PeriodIndex(["2011-01", "2011-02"], freq="M")
+        tdi = pd.TimedeltaIndex(["1 days", "2 days"])
+        exp = Index(
+            [
+                pd.Period("2011-01", freq="M"),
+                pd.Period("2011-02", freq="M"),
+                pd.Timedelta("1 days"),
+                pd.Timedelta("2 days"),
+            ],
+            dtype=object,
+        )
+
+        res = pi1.append(tdi)
+        tm.assert_index_equal(res, exp)
+
+        ps1 = Series(pi1)
+        tds = Series(tdi)
+        res = ps1._append(tds)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([ps1, tds])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        # inverse
+        exp = Index(
+            [
+                pd.Timedelta("1 days"),
+                pd.Timedelta("2 days"),
+                pd.Period("2011-01", freq="M"),
+                pd.Period("2011-02", freq="M"),
+            ],
+            dtype=object,
+        )
+
+        res = tdi.append(pi1)
+        tm.assert_index_equal(res, exp)
+
+        ps1 = Series(pi1)
+        tds = Series(tdi)
+        res = tds._append(ps1)
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+        res = pd.concat([tds, ps1])
+        tm.assert_series_equal(res, Series(exp, index=[0, 1, 0, 1]))
+
+    def test_concat_categorical(self):
+        # GH 13524
+
+        # same categories -> category
+        s1 = Series([1, 2, np.nan], dtype="category")
+        s2 = Series([2, 1, 2], dtype="category")
+
+        exp = Series([1, 2, np.nan, 2, 1, 2], dtype="category")
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        # partially different categories => not-category
+        s1 = Series([3, 2], dtype="category")
+        s2 = Series([2, 1], dtype="category")
+
+        exp = Series([3, 2, 2, 1])
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        # completely different categories (same dtype) => not-category
+        s1 = Series([10, 11, np.nan], dtype="category")
+        s2 = Series([np.nan, 1, 3, 2], dtype="category")
+
+        exp = Series([10, 11, np.nan, np.nan, 1, 3, 2], dtype=np.float64)
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+    def test_union_categorical_same_categories_different_order(self):
+        # https://github.com/pandas-dev/pandas/issues/19096
+        a = Series(Categorical(["a", "b", "c"], categories=["a", "b", "c"]))
+        b = Series(Categorical(["a", "b", "c"], categories=["b", "a", "c"]))
+        result = pd.concat([a, b], ignore_index=True)
+        expected = Series(
+            Categorical(["a", "b", "c", "a", "b", "c"], categories=["a", "b", "c"])
+        )
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_categorical_coercion(self):
+        # GH 13524
+
+        # category + not-category => not-category
+        s1 = Series([1, 2, np.nan], dtype="category")
+        s2 = Series([2, 1, 2])
+
+        exp = Series([1, 2, np.nan, 2, 1, 2], dtype=np.float64)
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        # result shouldn't be affected by 1st elem dtype
+        exp = Series([2, 1, 2, 1, 2, np.nan], dtype=np.float64)
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+        # all values are not in category => not-category
+        s1 = Series([3, 2], dtype="category")
+        s2 = Series([2, 1])
+
+        exp = Series([3, 2, 2, 1])
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        exp = Series([2, 1, 3, 2])
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+        # completely different categories => not-category
+        s1 = Series([10, 11, np.nan], dtype="category")
+        s2 = Series([1, 3, 2])
+
+        exp = Series([10, 11, np.nan, 1, 3, 2], dtype=np.float64)
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        exp = Series([1, 3, 2, 10, 11, np.nan], dtype=np.float64)
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+        # different dtype => not-category
+        s1 = Series([10, 11, np.nan], dtype="category")
+        s2 = Series(["a", "b", "c"])
+
+        exp = Series([10, 11, np.nan, "a", "b", "c"])
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        exp = Series(["a", "b", "c", 10, 11, np.nan])
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+        # if normal series only contains NaN-likes => not-category
+        s1 = Series([10, 11], dtype="category")
+        s2 = Series([np.nan, np.nan, np.nan])
+
+        exp = Series([10, 11, np.nan, np.nan, np.nan])
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        exp = Series([np.nan, np.nan, np.nan, 10, 11])
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+    def test_concat_categorical_3elem_coercion(self):
+        # GH 13524
+
+        # mixed dtypes => not-category
+        s1 = Series([1, 2, np.nan], dtype="category")
+        s2 = Series([2, 1, 2], dtype="category")
+        s3 = Series([1, 2, 1, 2, np.nan])
+
+        exp = Series([1, 2, np.nan, 2, 1, 2, 1, 2, 1, 2, np.nan], dtype="float")
+        tm.assert_series_equal(pd.concat([s1, s2, s3], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append([s2, s3], ignore_index=True), exp)
+
+        exp = Series([1, 2, 1, 2, np.nan, 1, 2, np.nan, 2, 1, 2], dtype="float")
+        tm.assert_series_equal(pd.concat([s3, s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s3._append([s1, s2], ignore_index=True), exp)
+
+        # values are all in either category => not-category
+        s1 = Series([4, 5, 6], dtype="category")
+        s2 = Series([1, 2, 3], dtype="category")
+        s3 = Series([1, 3, 4])
+
+        exp = Series([4, 5, 6, 1, 2, 3, 1, 3, 4])
+        tm.assert_series_equal(pd.concat([s1, s2, s3], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append([s2, s3], ignore_index=True), exp)
+
+        exp = Series([1, 3, 4, 4, 5, 6, 1, 2, 3])
+        tm.assert_series_equal(pd.concat([s3, s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s3._append([s1, s2], ignore_index=True), exp)
+
+        # values are all in either category => not-category
+        s1 = Series([4, 5, 6], dtype="category")
+        s2 = Series([1, 2, 3], dtype="category")
+        s3 = Series([10, 11, 12])
+
+        exp = Series([4, 5, 6, 1, 2, 3, 10, 11, 12])
+        tm.assert_series_equal(pd.concat([s1, s2, s3], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append([s2, s3], ignore_index=True), exp)
+
+        exp = Series([10, 11, 12, 4, 5, 6, 1, 2, 3])
+        tm.assert_series_equal(pd.concat([s3, s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s3._append([s1, s2], ignore_index=True), exp)
+
+    def test_concat_categorical_multi_coercion(self):
+        # GH 13524
+
+        s1 = Series([1, 3], dtype="category")
+        s2 = Series([3, 4], dtype="category")
+        s3 = Series([2, 3])
+        s4 = Series([2, 2], dtype="category")
+        s5 = Series([1, np.nan])
+        s6 = Series([1, 3, 2], dtype="category")
+
+        # mixed dtype, values are all in categories => not-category
+        exp = Series([1, 3, 3, 4, 2, 3, 2, 2, 1, np.nan, 1, 3, 2])
+        res = pd.concat([s1, s2, s3, s4, s5, s6], ignore_index=True)
+        tm.assert_series_equal(res, exp)
+        res = s1._append([s2, s3, s4, s5, s6], ignore_index=True)
+        tm.assert_series_equal(res, exp)
+
+        exp = Series([1, 3, 2, 1, np.nan, 2, 2, 2, 3, 3, 4, 1, 3])
+        res = pd.concat([s6, s5, s4, s3, s2, s1], ignore_index=True)
+        tm.assert_series_equal(res, exp)
+        res = s6._append([s5, s4, s3, s2, s1], ignore_index=True)
+        tm.assert_series_equal(res, exp)
+
+    def test_concat_categorical_ordered(self):
+        # GH 13524
+
+        s1 = Series(Categorical([1, 2, np.nan], ordered=True))
+        s2 = Series(Categorical([2, 1, 2], ordered=True))
+
+        exp = Series(Categorical([1, 2, np.nan, 2, 1, 2], ordered=True))
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        exp = Series(Categorical([1, 2, np.nan, 2, 1, 2, 1, 2, np.nan], ordered=True))
+        tm.assert_series_equal(pd.concat([s1, s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append([s2, s1], ignore_index=True), exp)
+
+    def test_concat_categorical_coercion_nan(self):
+        # GH 13524
+
+        # some edge cases
+        # category + not-category => not category
+        s1 = Series(np.array([np.nan, np.nan], dtype=np.float64), dtype="category")
+        s2 = Series([np.nan, 1])
+
+        exp = Series([np.nan, np.nan, np.nan, 1])
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        s1 = Series([1, np.nan], dtype="category")
+        s2 = Series([np.nan, np.nan])
+
+        exp = Series([1, np.nan, np.nan, np.nan], dtype="float")
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        # mixed dtype, all nan-likes => not-category
+        s1 = Series([np.nan, np.nan], dtype="category")
+        s2 = Series([np.nan, np.nan])
+
+        exp = Series([np.nan, np.nan, np.nan, np.nan])
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+        # all category nan-likes => category
+        s1 = Series([np.nan, np.nan], dtype="category")
+        s2 = Series([np.nan, np.nan], dtype="category")
+
+        exp = Series([np.nan, np.nan, np.nan, np.nan], dtype="category")
+
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+    def test_concat_categorical_empty(self):
+        # GH 13524
+
+        s1 = Series([], dtype="category")
+        s2 = Series([1, 2], dtype="category")
+
+        msg = "The behavior of array concatenation with empty entries is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), s2)
+            tm.assert_series_equal(s1._append(s2, ignore_index=True), s2)
+
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), s2)
+            tm.assert_series_equal(s2._append(s1, ignore_index=True), s2)
+
+        s1 = Series([], dtype="category")
+        s2 = Series([], dtype="category")
+
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), s2)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), s2)
+
+        s1 = Series([], dtype="category")
+        s2 = Series([], dtype="object")
+
+        # different dtype => not-category
+        tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), s2)
+        tm.assert_series_equal(s1._append(s2, ignore_index=True), s2)
+        tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), s2)
+        tm.assert_series_equal(s2._append(s1, ignore_index=True), s2)
+
+        s1 = Series([], dtype="category")
+        s2 = Series([np.nan, np.nan])
+
+        # empty Series is ignored
+        exp = Series([np.nan, np.nan])
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            tm.assert_series_equal(pd.concat([s1, s2], ignore_index=True), exp)
+            tm.assert_series_equal(s1._append(s2, ignore_index=True), exp)
+
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            tm.assert_series_equal(pd.concat([s2, s1], ignore_index=True), exp)
+            tm.assert_series_equal(s2._append(s1, ignore_index=True), exp)
+
+    def test_categorical_concat_append(self):
+        cat = Categorical(["a", "b"], categories=["a", "b"])
+        vals = [1, 2]
+        df = DataFrame({"cats": cat, "vals": vals})
+        cat2 = Categorical(["a", "b", "a", "b"], categories=["a", "b"])
+        vals2 = [1, 2, 1, 2]
+        exp = DataFrame({"cats": cat2, "vals": vals2}, index=Index([0, 1, 0, 1]))
+
+        tm.assert_frame_equal(pd.concat([df, df]), exp)
+        tm.assert_frame_equal(df._append(df), exp)
+
+        # GH 13524 can concat different categories
+        cat3 = Categorical(["a", "b"], categories=["a", "b", "c"])
+        vals3 = [1, 2]
+        df_different_categories = DataFrame({"cats": cat3, "vals": vals3})
+
+        res = pd.concat([df, df_different_categories], ignore_index=True)
+        exp = DataFrame({"cats": list("abab"), "vals": [1, 2, 1, 2]})
+        tm.assert_frame_equal(res, exp)
+
+        res = df._append(df_different_categories, ignore_index=True)
+        tm.assert_frame_equal(res, exp)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_categorical.py

@@ -0,0 +1,273 @@
+from datetime import datetime
+
+import numpy as np
+
+from pandas.core.dtypes.dtypes import CategoricalDtype
+
+import pandas as pd
+from pandas import (
+    Categorical,
+    DataFrame,
+    Series,
+)
+import pandas._testing as tm
+
+
+class TestCategoricalConcat:
+    def test_categorical_concat(self, sort):
+        # See GH 10177
+        df1 = DataFrame(
+            np.arange(18, dtype="int64").reshape(6, 3), columns=["a", "b", "c"]
+        )
+
+        df2 = DataFrame(np.arange(14, dtype="int64").reshape(7, 2), columns=["a", "c"])
+
+        cat_values = ["one", "one", "two", "one", "two", "two", "one"]
+        df2["h"] = Series(Categorical(cat_values))
+
+        res = pd.concat((df1, df2), axis=0, ignore_index=True, sort=sort)
+        exp = DataFrame(
+            {
+                "a": [0, 3, 6, 9, 12, 15, 0, 2, 4, 6, 8, 10, 12],
+                "b": [
+                    1,
+                    4,
+                    7,
+                    10,
+                    13,
+                    16,
+                    np.nan,
+                    np.nan,
+                    np.nan,
+                    np.nan,
+                    np.nan,
+                    np.nan,
+                    np.nan,
+                ],
+                "c": [2, 5, 8, 11, 14, 17, 1, 3, 5, 7, 9, 11, 13],
+                "h": [None] * 6 + cat_values,
+            }
+        )
+        exp["h"] = exp["h"].astype(df2["h"].dtype)
+        tm.assert_frame_equal(res, exp)
+
+    def test_categorical_concat_dtypes(self, using_infer_string):
+        # GH8143
+        index = ["cat", "obj", "num"]
+        cat = Categorical(["a", "b", "c"])
+        obj = Series(["a", "b", "c"])
+        num = Series([1, 2, 3])
+        df = pd.concat([Series(cat), obj, num], axis=1, keys=index)
+
+        result = df.dtypes == (
+            object if not using_infer_string else "string[pyarrow_numpy]"
+        )
+        expected = Series([False, True, False], index=index)
+        tm.assert_series_equal(result, expected)
+
+        result = df.dtypes == "int64"
+        expected = Series([False, False, True], index=index)
+        tm.assert_series_equal(result, expected)
+
+        result = df.dtypes == "category"
+        expected = Series([True, False, False], index=index)
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_categoricalindex(self):
+        # GH 16111, categories that aren't lexsorted
+        categories = [9, 0, 1, 2, 3]
+
+        a = Series(1, index=pd.CategoricalIndex([9, 0], categories=categories))
+        b = Series(2, index=pd.CategoricalIndex([0, 1], categories=categories))
+        c = Series(3, index=pd.CategoricalIndex([1, 2], categories=categories))
+
+        result = pd.concat([a, b, c], axis=1)
+
+        exp_idx = pd.CategoricalIndex([9, 0, 1, 2], categories=categories)
+        exp = DataFrame(
+            {
+                0: [1, 1, np.nan, np.nan],
+                1: [np.nan, 2, 2, np.nan],
+                2: [np.nan, np.nan, 3, 3],
+            },
+            columns=[0, 1, 2],
+            index=exp_idx,
+        )
+        tm.assert_frame_equal(result, exp)
+
+    def test_categorical_concat_preserve(self):
+        # GH 8641  series concat not preserving category dtype
+        # GH 13524 can concat different categories
+        s = Series(list("abc"), dtype="category")
+        s2 = Series(list("abd"), dtype="category")
+
+        exp = Series(list("abcabd"))
+        res = pd.concat([s, s2], ignore_index=True)
+        tm.assert_series_equal(res, exp)
+
+        exp = Series(list("abcabc"), dtype="category")
+        res = pd.concat([s, s], ignore_index=True)
+        tm.assert_series_equal(res, exp)
+
+        exp = Series(list("abcabc"), index=[0, 1, 2, 0, 1, 2], dtype="category")
+        res = pd.concat([s, s])
+        tm.assert_series_equal(res, exp)
+
+        a = Series(np.arange(6, dtype="int64"))
+        b = Series(list("aabbca"))
+
+        df2 = DataFrame({"A": a, "B": b.astype(CategoricalDtype(list("cab")))})
+        res = pd.concat([df2, df2])
+        exp = DataFrame(
+            {
+                "A": pd.concat([a, a]),
+                "B": pd.concat([b, b]).astype(CategoricalDtype(list("cab"))),
+            }
+        )
+        tm.assert_frame_equal(res, exp)
+
+    def test_categorical_index_preserver(self):
+        a = Series(np.arange(6, dtype="int64"))
+        b = Series(list("aabbca"))
+
+        df2 = DataFrame(
+            {"A": a, "B": b.astype(CategoricalDtype(list("cab")))}
+        ).set_index("B")
+        result = pd.concat([df2, df2])
+        expected = DataFrame(
+            {
+                "A": pd.concat([a, a]),
+                "B": pd.concat([b, b]).astype(CategoricalDtype(list("cab"))),
+            }
+        ).set_index("B")
+        tm.assert_frame_equal(result, expected)
+
+        # wrong categories -> uses concat_compat, which casts to object
+        df3 = DataFrame(
+            {"A": a, "B": Categorical(b, categories=list("abe"))}
+        ).set_index("B")
+        result = pd.concat([df2, df3])
+        expected = pd.concat(
+            [
+                df2.set_axis(df2.index.astype(object), axis=0),
+                df3.set_axis(df3.index.astype(object), axis=0),
+            ]
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_categorical_tz(self):
+        # GH-23816
+        a = Series(pd.date_range("2017-01-01", periods=2, tz="US/Pacific"))
+        b = Series(["a", "b"], dtype="category")
+        result = pd.concat([a, b], ignore_index=True)
+        expected = Series(
+            [
+                pd.Timestamp("2017-01-01", tz="US/Pacific"),
+                pd.Timestamp("2017-01-02", tz="US/Pacific"),
+                "a",
+                "b",
+            ]
+        )
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_categorical_datetime(self):
+        # GH-39443
+        df1 = DataFrame(
+            {"x": Series(datetime(2021, 1, 1), index=[0], dtype="category")}
+        )
+        df2 = DataFrame(
+            {"x": Series(datetime(2021, 1, 2), index=[1], dtype="category")}
+        )
+
+        result = pd.concat([df1, df2])
+        expected = DataFrame(
+            {"x": Series([datetime(2021, 1, 1), datetime(2021, 1, 2)])}
+        )
+
+        tm.assert_equal(result, expected)
+
+    def test_concat_categorical_unchanged(self):
+        # GH-12007
+        # test fix for when concat on categorical and float
+        # coerces dtype categorical -> float
+        df = DataFrame(Series(["a", "b", "c"], dtype="category", name="A"))
+        ser = Series([0, 1, 2], index=[0, 1, 3], name="B")
+        result = pd.concat([df, ser], axis=1)
+        expected = DataFrame(
+            {
+                "A": Series(["a", "b", "c", np.nan], dtype="category"),
+                "B": Series([0, 1, np.nan, 2], dtype="float"),
+            }
+        )
+        tm.assert_equal(result, expected)
+
+    def test_categorical_concat_gh7864(self):
+        # GH 7864
+        # make sure ordering is preserved
+        df = DataFrame({"id": [1, 2, 3, 4, 5, 6], "raw_grade": list("abbaae")})
+        df["grade"] = Categorical(df["raw_grade"])
+        df["grade"].cat.set_categories(["e", "a", "b"])
+
+        df1 = df[0:3]
+        df2 = df[3:]
+
+        tm.assert_index_equal(df["grade"].cat.categories, df1["grade"].cat.categories)
+        tm.assert_index_equal(df["grade"].cat.categories, df2["grade"].cat.categories)
+
+        dfx = pd.concat([df1, df2])
+        tm.assert_index_equal(df["grade"].cat.categories, dfx["grade"].cat.categories)
+
+        dfa = df1._append(df2)
+        tm.assert_index_equal(df["grade"].cat.categories, dfa["grade"].cat.categories)
+
+    def test_categorical_index_upcast(self):
+        # GH 17629
+        # test upcasting to object when concatenating on categorical indexes
+        # with non-identical categories
+
+        a = DataFrame({"foo": [1, 2]}, index=Categorical(["foo", "bar"]))
+        b = DataFrame({"foo": [4, 3]}, index=Categorical(["baz", "bar"]))
+
+        res = pd.concat([a, b])
+        exp = DataFrame({"foo": [1, 2, 4, 3]}, index=["foo", "bar", "baz", "bar"])
+
+        tm.assert_equal(res, exp)
+
+        a = Series([1, 2], index=Categorical(["foo", "bar"]))
+        b = Series([4, 3], index=Categorical(["baz", "bar"]))
+
+        res = pd.concat([a, b])
+        exp = Series([1, 2, 4, 3], index=["foo", "bar", "baz", "bar"])
+
+        tm.assert_equal(res, exp)
+
+    def test_categorical_missing_from_one_frame(self):
+        # GH 25412
+        df1 = DataFrame({"f1": [1, 2, 3]})
+        df2 = DataFrame({"f1": [2, 3, 1], "f2": Series([4, 4, 4]).astype("category")})
+        result = pd.concat([df1, df2], sort=True)
+        dtype = CategoricalDtype([4])
+        expected = DataFrame(
+            {
+                "f1": [1, 2, 3, 2, 3, 1],
+                "f2": Categorical.from_codes([-1, -1, -1, 0, 0, 0], dtype=dtype),
+            },
+            index=[0, 1, 2, 0, 1, 2],
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_categorical_same_categories_different_order(self):
+        # https://github.com/pandas-dev/pandas/issues/24845
+
+        c1 = pd.CategoricalIndex(["a", "a"], categories=["a", "b"], ordered=False)
+        c2 = pd.CategoricalIndex(["b", "b"], categories=["b", "a"], ordered=False)
+        c3 = pd.CategoricalIndex(
+            ["a", "a", "b", "b"], categories=["a", "b"], ordered=False
+        )
+
+        df1 = DataFrame({"A": [1, 2]}, index=c1)
+        df2 = DataFrame({"A": [3, 4]}, index=c2)
+
+        result = pd.concat((df1, df2))
+        expected = DataFrame({"A": [1, 2, 3, 4]}, index=c3)
+        tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_concat.py

@@ -0,0 +1,912 @@
+from collections import (
+    abc,
+    deque,
+)
+from collections.abc import Iterator
+from datetime import datetime
+from decimal import Decimal
+
+import numpy as np
+import pytest
+
+from pandas.errors import InvalidIndexError
+import pandas.util._test_decorators as td
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    Index,
+    MultiIndex,
+    PeriodIndex,
+    Series,
+    concat,
+    date_range,
+)
+import pandas._testing as tm
+from pandas.core.arrays import SparseArray
+from pandas.tests.extension.decimal import to_decimal
+
+
+class TestConcatenate:
+    def test_append_concat(self):
+        # GH#1815
+        d1 = date_range("12/31/1990", "12/31/1999", freq="YE-DEC")
+        d2 = date_range("12/31/2000", "12/31/2009", freq="YE-DEC")
+
+        s1 = Series(np.random.default_rng(2).standard_normal(10), d1)
+        s2 = Series(np.random.default_rng(2).standard_normal(10), d2)
+
+        s1 = s1.to_period()
+        s2 = s2.to_period()
+
+        # drops index
+        result = concat([s1, s2])
+        assert isinstance(result.index, PeriodIndex)
+        assert result.index[0] == s1.index[0]
+
+    def test_concat_copy(self, using_array_manager, using_copy_on_write):
+        df = DataFrame(np.random.default_rng(2).standard_normal((4, 3)))
+        df2 = DataFrame(np.random.default_rng(2).integers(0, 10, size=4).reshape(4, 1))
+        df3 = DataFrame({5: "foo"}, index=range(4))
+
+        # These are actual copies.
+        result = concat([df, df2, df3], axis=1, copy=True)
+
+        if not using_copy_on_write:
+            for arr in result._mgr.arrays:
+                assert not any(
+                    np.shares_memory(arr, y)
+                    for x in [df, df2, df3]
+                    for y in x._mgr.arrays
+                )
+        else:
+            for arr in result._mgr.arrays:
+                assert arr.base is not None
+
+        # These are the same.
+        result = concat([df, df2, df3], axis=1, copy=False)
+
+        for arr in result._mgr.arrays:
+            if arr.dtype.kind == "f":
+                assert arr.base is df._mgr.arrays[0].base
+            elif arr.dtype.kind in ["i", "u"]:
+                assert arr.base is df2._mgr.arrays[0].base
+            elif arr.dtype == object:
+                if using_array_manager:
+                    # we get the same array object, which has no base
+                    assert arr is df3._mgr.arrays[0]
+                else:
+                    assert arr.base is not None
+
+        # Float block was consolidated.
+        df4 = DataFrame(np.random.default_rng(2).standard_normal((4, 1)))
+        result = concat([df, df2, df3, df4], axis=1, copy=False)
+        for arr in result._mgr.arrays:
+            if arr.dtype.kind == "f":
+                if using_array_manager or using_copy_on_write:
+                    # this is a view on some array in either df or df4
+                    assert any(
+                        np.shares_memory(arr, other)
+                        for other in df._mgr.arrays + df4._mgr.arrays
+                    )
+                else:
+                    # the block was consolidated, so we got a copy anyway
+                    assert arr.base is None
+            elif arr.dtype.kind in ["i", "u"]:
+                assert arr.base is df2._mgr.arrays[0].base
+            elif arr.dtype == object:
+                # this is a view on df3
+                assert any(np.shares_memory(arr, other) for other in df3._mgr.arrays)
+
+    def test_concat_with_group_keys(self):
+        # axis=0
+        df = DataFrame(np.random.default_rng(2).standard_normal((3, 4)))
+        df2 = DataFrame(np.random.default_rng(2).standard_normal((4, 4)))
+
+        result = concat([df, df2], keys=[0, 1])
+        exp_index = MultiIndex.from_arrays(
+            [[0, 0, 0, 1, 1, 1, 1], [0, 1, 2, 0, 1, 2, 3]]
+        )
+        expected = DataFrame(np.r_[df.values, df2.values], index=exp_index)
+        tm.assert_frame_equal(result, expected)
+
+        result = concat([df, df], keys=[0, 1])
+        exp_index2 = MultiIndex.from_arrays([[0, 0, 0, 1, 1, 1], [0, 1, 2, 0, 1, 2]])
+        expected = DataFrame(np.r_[df.values, df.values], index=exp_index2)
+        tm.assert_frame_equal(result, expected)
+
+        # axis=1
+        df = DataFrame(np.random.default_rng(2).standard_normal((4, 3)))
+        df2 = DataFrame(np.random.default_rng(2).standard_normal((4, 4)))
+
+        result = concat([df, df2], keys=[0, 1], axis=1)
+        expected = DataFrame(np.c_[df.values, df2.values], columns=exp_index)
+        tm.assert_frame_equal(result, expected)
+
+        result = concat([df, df], keys=[0, 1], axis=1)
+        expected = DataFrame(np.c_[df.values, df.values], columns=exp_index2)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_keys_specific_levels(self):
+        df = DataFrame(np.random.default_rng(2).standard_normal((10, 4)))
+        pieces = [df.iloc[:, [0, 1]], df.iloc[:, [2]], df.iloc[:, [3]]]
+        level = ["three", "two", "one", "zero"]
+        result = concat(
+            pieces,
+            axis=1,
+            keys=["one", "two", "three"],
+            levels=[level],
+            names=["group_key"],
+        )
+
+        tm.assert_index_equal(result.columns.levels[0], Index(level, name="group_key"))
+        tm.assert_index_equal(result.columns.levels[1], Index([0, 1, 2, 3]))
+
+        assert result.columns.names == ["group_key", None]
+
+    @pytest.mark.parametrize("mapping", ["mapping", "dict"])
+    def test_concat_mapping(self, mapping, non_dict_mapping_subclass):
+        constructor = dict if mapping == "dict" else non_dict_mapping_subclass
+        frames = constructor(
+            {
+                "foo": DataFrame(np.random.default_rng(2).standard_normal((4, 3))),
+                "bar": DataFrame(np.random.default_rng(2).standard_normal((4, 3))),
+                "baz": DataFrame(np.random.default_rng(2).standard_normal((4, 3))),
+                "qux": DataFrame(np.random.default_rng(2).standard_normal((4, 3))),
+            }
+        )
+
+        sorted_keys = list(frames.keys())
+
+        result = concat(frames)
+        expected = concat([frames[k] for k in sorted_keys], keys=sorted_keys)
+        tm.assert_frame_equal(result, expected)
+
+        result = concat(frames, axis=1)
+        expected = concat([frames[k] for k in sorted_keys], keys=sorted_keys, axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        keys = ["baz", "foo", "bar"]
+        result = concat(frames, keys=keys)
+        expected = concat([frames[k] for k in keys], keys=keys)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_keys_and_levels(self):
+        df = DataFrame(np.random.default_rng(2).standard_normal((1, 3)))
+        df2 = DataFrame(np.random.default_rng(2).standard_normal((1, 4)))
+
+        levels = [["foo", "baz"], ["one", "two"]]
+        names = ["first", "second"]
+        result = concat(
+            [df, df2, df, df2],
+            keys=[("foo", "one"), ("foo", "two"), ("baz", "one"), ("baz", "two")],
+            levels=levels,
+            names=names,
+        )
+        expected = concat([df, df2, df, df2])
+        exp_index = MultiIndex(
+            levels=levels + [[0]],
+            codes=[[0, 0, 1, 1], [0, 1, 0, 1], [0, 0, 0, 0]],
+            names=names + [None],
+        )
+        expected.index = exp_index
+
+        tm.assert_frame_equal(result, expected)
+
+        # no names
+        result = concat(
+            [df, df2, df, df2],
+            keys=[("foo", "one"), ("foo", "two"), ("baz", "one"), ("baz", "two")],
+            levels=levels,
+        )
+        assert result.index.names == (None,) * 3
+
+        # no levels
+        result = concat(
+            [df, df2, df, df2],
+            keys=[("foo", "one"), ("foo", "two"), ("baz", "one"), ("baz", "two")],
+            names=["first", "second"],
+        )
+        assert result.index.names == ("first", "second", None)
+        tm.assert_index_equal(
+            result.index.levels[0], Index(["baz", "foo"], name="first")
+        )
+
+    def test_concat_keys_levels_no_overlap(self):
+        # GH #1406
+        df = DataFrame(np.random.default_rng(2).standard_normal((1, 3)), index=["a"])
+        df2 = DataFrame(np.random.default_rng(2).standard_normal((1, 4)), index=["b"])
+
+        msg = "Values not found in passed level"
+        with pytest.raises(ValueError, match=msg):
+            concat([df, df], keys=["one", "two"], levels=[["foo", "bar", "baz"]])
+
+        msg = "Key one not in level"
+        with pytest.raises(ValueError, match=msg):
+            concat([df, df2], keys=["one", "two"], levels=[["foo", "bar", "baz"]])
+
+    def test_crossed_dtypes_weird_corner(self):
+        columns = ["A", "B", "C", "D"]
+        df1 = DataFrame(
+            {
+                "A": np.array([1, 2, 3, 4], dtype="f8"),
+                "B": np.array([1, 2, 3, 4], dtype="i8"),
+                "C": np.array([1, 2, 3, 4], dtype="f8"),
+                "D": np.array([1, 2, 3, 4], dtype="i8"),
+            },
+            columns=columns,
+        )
+
+        df2 = DataFrame(
+            {
+                "A": np.array([1, 2, 3, 4], dtype="i8"),
+                "B": np.array([1, 2, 3, 4], dtype="f8"),
+                "C": np.array([1, 2, 3, 4], dtype="i8"),
+                "D": np.array([1, 2, 3, 4], dtype="f8"),
+            },
+            columns=columns,
+        )
+
+        appended = concat([df1, df2], ignore_index=True)
+        expected = DataFrame(
+            np.concatenate([df1.values, df2.values], axis=0), columns=columns
+        )
+        tm.assert_frame_equal(appended, expected)
+
+        df = DataFrame(np.random.default_rng(2).standard_normal((1, 3)), index=["a"])
+        df2 = DataFrame(np.random.default_rng(2).standard_normal((1, 4)), index=["b"])
+        result = concat([df, df2], keys=["one", "two"], names=["first", "second"])
+        assert result.index.names == ("first", "second")
+
+    def test_with_mixed_tuples(self, sort):
+        # 10697
+        # columns have mixed tuples, so handle properly
+        df1 = DataFrame({"A": "foo", ("B", 1): "bar"}, index=range(2))
+        df2 = DataFrame({"B": "foo", ("B", 1): "bar"}, index=range(2))
+
+        # it works
+        concat([df1, df2], sort=sort)
+
+    def test_concat_mixed_objs_columns(self):
+        # Test column-wise concat for mixed series/frames (axis=1)
+        # G2385
+
+        index = date_range("01-Jan-2013", periods=10, freq="h")
+        arr = np.arange(10, dtype="int64")
+        s1 = Series(arr, index=index)
+        s2 = Series(arr, index=index)
+        df = DataFrame(arr.reshape(-1, 1), index=index)
+
+        expected = DataFrame(
+            np.repeat(arr, 2).reshape(-1, 2), index=index, columns=[0, 0]
+        )
+        result = concat([df, df], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        expected = DataFrame(
+            np.repeat(arr, 2).reshape(-1, 2), index=index, columns=[0, 1]
+        )
+        result = concat([s1, s2], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        expected = DataFrame(
+            np.repeat(arr, 3).reshape(-1, 3), index=index, columns=[0, 1, 2]
+        )
+        result = concat([s1, s2, s1], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        expected = DataFrame(
+            np.repeat(arr, 5).reshape(-1, 5), index=index, columns=[0, 0, 1, 2, 3]
+        )
+        result = concat([s1, df, s2, s2, s1], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        # with names
+        s1.name = "foo"
+        expected = DataFrame(
+            np.repeat(arr, 3).reshape(-1, 3), index=index, columns=["foo", 0, 0]
+        )
+        result = concat([s1, df, s2], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        s2.name = "bar"
+        expected = DataFrame(
+            np.repeat(arr, 3).reshape(-1, 3), index=index, columns=["foo", 0, "bar"]
+        )
+        result = concat([s1, df, s2], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        # ignore index
+        expected = DataFrame(
+            np.repeat(arr, 3).reshape(-1, 3), index=index, columns=[0, 1, 2]
+        )
+        result = concat([s1, df, s2], axis=1, ignore_index=True)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_mixed_objs_index(self):
+        # Test row-wise concat for mixed series/frames with a common name
+        # GH2385, GH15047
+
+        index = date_range("01-Jan-2013", periods=10, freq="h")
+        arr = np.arange(10, dtype="int64")
+        s1 = Series(arr, index=index)
+        s2 = Series(arr, index=index)
+        df = DataFrame(arr.reshape(-1, 1), index=index)
+
+        expected = DataFrame(
+            np.tile(arr, 3).reshape(-1, 1), index=index.tolist() * 3, columns=[0]
+        )
+        result = concat([s1, df, s2])
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_mixed_objs_index_names(self):
+        # Test row-wise concat for mixed series/frames with distinct names
+        # GH2385, GH15047
+
+        index = date_range("01-Jan-2013", periods=10, freq="h")
+        arr = np.arange(10, dtype="int64")
+        s1 = Series(arr, index=index, name="foo")
+        s2 = Series(arr, index=index, name="bar")
+        df = DataFrame(arr.reshape(-1, 1), index=index)
+
+        expected = DataFrame(
+            np.kron(np.where(np.identity(3) == 1, 1, np.nan), arr).T,
+            index=index.tolist() * 3,
+            columns=["foo", 0, "bar"],
+        )
+        result = concat([s1, df, s2])
+        tm.assert_frame_equal(result, expected)
+
+        # Rename all series to 0 when ignore_index=True
+        expected = DataFrame(np.tile(arr, 3).reshape(-1, 1), columns=[0])
+        result = concat([s1, df, s2], ignore_index=True)
+        tm.assert_frame_equal(result, expected)
+
+    def test_dtype_coercion(self):
+        # 12411
+        df = DataFrame({"date": [pd.Timestamp("20130101").tz_localize("UTC"), pd.NaT]})
+
+        result = concat([df.iloc[[0]], df.iloc[[1]]])
+        tm.assert_series_equal(result.dtypes, df.dtypes)
+
+        # 12045
+        df = DataFrame({"date": [datetime(2012, 1, 1), datetime(1012, 1, 2)]})
+        result = concat([df.iloc[[0]], df.iloc[[1]]])
+        tm.assert_series_equal(result.dtypes, df.dtypes)
+
+        # 11594
+        df = DataFrame({"text": ["some words"] + [None] * 9})
+        result = concat([df.iloc[[0]], df.iloc[[1]]])
+        tm.assert_series_equal(result.dtypes, df.dtypes)
+
+    def test_concat_single_with_key(self):
+        df = DataFrame(np.random.default_rng(2).standard_normal((10, 4)))
+
+        result = concat([df], keys=["foo"])
+        expected = concat([df, df], keys=["foo", "bar"])
+        tm.assert_frame_equal(result, expected[:10])
+
+    def test_concat_no_items_raises(self):
+        with pytest.raises(ValueError, match="No objects to concatenate"):
+            concat([])
+
+    def test_concat_exclude_none(self):
+        df = DataFrame(np.random.default_rng(2).standard_normal((10, 4)))
+
+        pieces = [df[:5], None, None, df[5:]]
+        result = concat(pieces)
+        tm.assert_frame_equal(result, df)
+        with pytest.raises(ValueError, match="All objects passed were None"):
+            concat([None, None])
+
+    def test_concat_keys_with_none(self):
+        # #1649
+        df0 = DataFrame([[10, 20, 30], [10, 20, 30], [10, 20, 30]])
+
+        result = concat({"a": None, "b": df0, "c": df0[:2], "d": df0[:1], "e": df0})
+        expected = concat({"b": df0, "c": df0[:2], "d": df0[:1], "e": df0})
+        tm.assert_frame_equal(result, expected)
+
+        result = concat(
+            [None, df0, df0[:2], df0[:1], df0], keys=["a", "b", "c", "d", "e"]
+        )
+        expected = concat([df0, df0[:2], df0[:1], df0], keys=["b", "c", "d", "e"])
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_bug_1719(self):
+        ts1 = Series(
+            np.arange(10, dtype=np.float64), index=date_range("2020-01-01", periods=10)
+        )
+        ts2 = ts1.copy()[::2]
+
+        # to join with union
+        # these two are of different length!
+        left = concat([ts1, ts2], join="outer", axis=1)
+        right = concat([ts2, ts1], join="outer", axis=1)
+
+        assert len(left) == len(right)
+
+    def test_concat_bug_2972(self):
+        ts0 = Series(np.zeros(5))
+        ts1 = Series(np.ones(5))
+        ts0.name = ts1.name = "same name"
+        result = concat([ts0, ts1], axis=1)
+
+        expected = DataFrame({0: ts0, 1: ts1})
+        expected.columns = ["same name", "same name"]
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_bug_3602(self):
+        # GH 3602, duplicate columns
+        df1 = DataFrame(
+            {
+                "firmNo": [0, 0, 0, 0],
+                "prc": [6, 6, 6, 6],
+                "stringvar": ["rrr", "rrr", "rrr", "rrr"],
+            }
+        )
+        df2 = DataFrame(
+            {"C": [9, 10, 11, 12], "misc": [1, 2, 3, 4], "prc": [6, 6, 6, 6]}
+        )
+        expected = DataFrame(
+            [
+                [0, 6, "rrr", 9, 1, 6],
+                [0, 6, "rrr", 10, 2, 6],
+                [0, 6, "rrr", 11, 3, 6],
+                [0, 6, "rrr", 12, 4, 6],
+            ]
+        )
+        expected.columns = ["firmNo", "prc", "stringvar", "C", "misc", "prc"]
+
+        result = concat([df1, df2], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_iterables(self):
+        # GH8645 check concat works with tuples, list, generators, and weird
+        # stuff like deque and custom iterables
+        df1 = DataFrame([1, 2, 3])
+        df2 = DataFrame([4, 5, 6])
+        expected = DataFrame([1, 2, 3, 4, 5, 6])
+        tm.assert_frame_equal(concat((df1, df2), ignore_index=True), expected)
+        tm.assert_frame_equal(concat([df1, df2], ignore_index=True), expected)
+        tm.assert_frame_equal(
+            concat((df for df in (df1, df2)), ignore_index=True), expected
+        )
+        tm.assert_frame_equal(concat(deque((df1, df2)), ignore_index=True), expected)
+
+        class CustomIterator1:
+            def __len__(self) -> int:
+                return 2
+
+            def __getitem__(self, index):
+                try:
+                    return {0: df1, 1: df2}[index]
+                except KeyError as err:
+                    raise IndexError from err
+
+        tm.assert_frame_equal(concat(CustomIterator1(), ignore_index=True), expected)
+
+        class CustomIterator2(abc.Iterable):
+            def __iter__(self) -> Iterator:
+                yield df1
+                yield df2
+
+        tm.assert_frame_equal(concat(CustomIterator2(), ignore_index=True), expected)
+
+    def test_concat_order(self):
+        # GH 17344, GH#47331
+        dfs = [DataFrame(index=range(3), columns=["a", 1, None])]
+        dfs += [DataFrame(index=range(3), columns=[None, 1, "a"]) for _ in range(100)]
+
+        result = concat(dfs, sort=True).columns
+        expected = Index([1, "a", None])
+        tm.assert_index_equal(result, expected)
+
+    def test_concat_different_extension_dtypes_upcasts(self):
+        a = Series(pd.array([1, 2], dtype="Int64"))
+        b = Series(to_decimal([1, 2]))
+
+        result = concat([a, b], ignore_index=True)
+        expected = Series([1, 2, Decimal(1), Decimal(2)], dtype=object)
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_ordered_dict(self):
+        # GH 21510
+        expected = concat(
+            [Series(range(3)), Series(range(4))], keys=["First", "Another"]
+        )
+        result = concat({"First": Series(range(3)), "Another": Series(range(4))})
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_duplicate_indices_raise(self):
+        # GH 45888: test raise for concat DataFrames with duplicate indices
+        # https://github.com/pandas-dev/pandas/issues/36263
+        df1 = DataFrame(
+            np.random.default_rng(2).standard_normal(5),
+            index=[0, 1, 2, 3, 3],
+            columns=["a"],
+        )
+        df2 = DataFrame(
+            np.random.default_rng(2).standard_normal(5),
+            index=[0, 1, 2, 2, 4],
+            columns=["b"],
+        )
+        msg = "Reindexing only valid with uniquely valued Index objects"
+        with pytest.raises(InvalidIndexError, match=msg):
+            concat([df1, df2], axis=1)
+
+
+def test_concat_no_unnecessary_upcast(float_numpy_dtype, frame_or_series):
+    # GH 13247
+    dims = frame_or_series(dtype=object).ndim
+    dt = float_numpy_dtype
+
+    dfs = [
+        frame_or_series(np.array([1], dtype=dt, ndmin=dims)),
+        frame_or_series(np.array([np.nan], dtype=dt, ndmin=dims)),
+        frame_or_series(np.array([5], dtype=dt, ndmin=dims)),
+    ]
+    x = concat(dfs)
+    assert x.values.dtype == dt
+
+
+@pytest.mark.parametrize("pdt", [Series, DataFrame])
+def test_concat_will_upcast(pdt, any_signed_int_numpy_dtype):
+    dt = any_signed_int_numpy_dtype
+    dims = pdt().ndim
+    dfs = [
+        pdt(np.array([1], dtype=dt, ndmin=dims)),
+        pdt(np.array([np.nan], ndmin=dims)),
+        pdt(np.array([5], dtype=dt, ndmin=dims)),
+    ]
+    x = concat(dfs)
+    assert x.values.dtype == "float64"
+
+
+def test_concat_empty_and_non_empty_frame_regression():
+    # GH 18178 regression test
+    df1 = DataFrame({"foo": [1]})
+    df2 = DataFrame({"foo": []})
+    expected = DataFrame({"foo": [1.0]})
+    result = concat([df1, df2])
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_sparse():
+    # GH 23557
+    a = Series(SparseArray([0, 1, 2]))
+    expected = DataFrame(data=[[0, 0], [1, 1], [2, 2]]).astype(
+        pd.SparseDtype(np.int64, 0)
+    )
+    result = concat([a, a], axis=1)
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_dense_sparse():
+    # GH 30668
+    dtype = pd.SparseDtype(np.float64, None)
+    a = Series(pd.arrays.SparseArray([1, None]), dtype=dtype)
+    b = Series([1], dtype=float)
+    expected = Series(data=[1, None, 1], index=[0, 1, 0]).astype(dtype)
+    result = concat([a, b], axis=0)
+    tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("keys", [["e", "f", "f"], ["f", "e", "f"]])
+def test_duplicate_keys(keys):
+    # GH 33654
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    s1 = Series([7, 8, 9], name="c")
+    s2 = Series([10, 11, 12], name="d")
+    result = concat([df, s1, s2], axis=1, keys=keys)
+    expected_values = [[1, 4, 7, 10], [2, 5, 8, 11], [3, 6, 9, 12]]
+    expected_columns = MultiIndex.from_tuples(
+        [(keys[0], "a"), (keys[0], "b"), (keys[1], "c"), (keys[2], "d")]
+    )
+    expected = DataFrame(expected_values, columns=expected_columns)
+    tm.assert_frame_equal(result, expected)
+
+
+def test_duplicate_keys_same_frame():
+    # GH 43595
+    keys = ["e", "e"]
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    result = concat([df, df], axis=1, keys=keys)
+    expected_values = [[1, 4, 1, 4], [2, 5, 2, 5], [3, 6, 3, 6]]
+    expected_columns = MultiIndex.from_tuples(
+        [(keys[0], "a"), (keys[0], "b"), (keys[1], "a"), (keys[1], "b")]
+    )
+    expected = DataFrame(expected_values, columns=expected_columns)
+    tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.filterwarnings(
+    "ignore:Passing a BlockManager|Passing a SingleBlockManager:DeprecationWarning"
+)
+@pytest.mark.parametrize(
+    "obj",
+    [
+        tm.SubclassedDataFrame({"A": np.arange(0, 10)}),
+        tm.SubclassedSeries(np.arange(0, 10), name="A"),
+    ],
+)
+def test_concat_preserves_subclass(obj):
+    # GH28330 -- preserve subclass
+
+    result = concat([obj, obj])
+    assert isinstance(result, type(obj))
+
+
+def test_concat_frame_axis0_extension_dtypes():
+    # preserve extension dtype (through common_dtype mechanism)
+    df1 = DataFrame({"a": pd.array([1, 2, 3], dtype="Int64")})
+    df2 = DataFrame({"a": np.array([4, 5, 6])})
+
+    result = concat([df1, df2], ignore_index=True)
+    expected = DataFrame({"a": [1, 2, 3, 4, 5, 6]}, dtype="Int64")
+    tm.assert_frame_equal(result, expected)
+
+    result = concat([df2, df1], ignore_index=True)
+    expected = DataFrame({"a": [4, 5, 6, 1, 2, 3]}, dtype="Int64")
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_preserves_extension_int64_dtype():
+    # GH 24768
+    df_a = DataFrame({"a": [-1]}, dtype="Int64")
+    df_b = DataFrame({"b": [1]}, dtype="Int64")
+    result = concat([df_a, df_b], ignore_index=True)
+    expected = DataFrame({"a": [-1, None], "b": [None, 1]}, dtype="Int64")
+    tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "dtype1,dtype2,expected_dtype",
+    [
+        ("bool", "bool", "bool"),
+        ("boolean", "bool", "boolean"),
+        ("bool", "boolean", "boolean"),
+        ("boolean", "boolean", "boolean"),
+    ],
+)
+def test_concat_bool_types(dtype1, dtype2, expected_dtype):
+    # GH 42800
+    ser1 = Series([True, False], dtype=dtype1)
+    ser2 = Series([False, True], dtype=dtype2)
+    result = concat([ser1, ser2], ignore_index=True)
+    expected = Series([True, False, False, True], dtype=expected_dtype)
+    tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    ("keys", "integrity"),
+    [
+        (["red"] * 3, True),
+        (["red"] * 3, False),
+        (["red", "blue", "red"], False),
+        (["red", "blue", "red"], True),
+    ],
+)
+def test_concat_repeated_keys(keys, integrity):
+    # GH: 20816
+    series_list = [Series({"a": 1}), Series({"b": 2}), Series({"c": 3})]
+    result = concat(series_list, keys=keys, verify_integrity=integrity)
+    tuples = list(zip(keys, ["a", "b", "c"]))
+    expected = Series([1, 2, 3], index=MultiIndex.from_tuples(tuples))
+    tm.assert_series_equal(result, expected)
+
+
+def test_concat_null_object_with_dti():
+    # GH#40841
+    dti = pd.DatetimeIndex(
+        ["2021-04-08 21:21:14+00:00"], dtype="datetime64[ns, UTC]", name="Time (UTC)"
+    )
+    right = DataFrame(data={"C": [0.5274]}, index=dti)
+
+    idx = Index([None], dtype="object", name="Maybe Time (UTC)")
+    left = DataFrame(data={"A": [None], "B": [np.nan]}, index=idx)
+
+    result = concat([left, right], axis="columns")
+
+    exp_index = Index([None, dti[0]], dtype=object)
+    expected = DataFrame(
+        {
+            "A": np.array([None, np.nan], dtype=object),
+            "B": [np.nan, np.nan],
+            "C": [np.nan, 0.5274],
+        },
+        index=exp_index,
+    )
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_multiindex_with_empty_rangeindex():
+    # GH#41234
+    mi = MultiIndex.from_tuples([("B", 1), ("C", 1)])
+    df1 = DataFrame([[1, 2]], columns=mi)
+    df2 = DataFrame(index=[1], columns=pd.RangeIndex(0))
+
+    result = concat([df1, df2])
+    expected = DataFrame([[1, 2], [np.nan, np.nan]], columns=mi)
+    tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        Series(data=[1, 2]),
+        DataFrame(
+            data={
+                "col1": [1, 2],
+            }
+        ),
+        DataFrame(dtype=float),
+        Series(dtype=float),
+    ],
+)
+def test_concat_drop_attrs(data):
+    # GH#41828
+    df1 = data.copy()
+    df1.attrs = {1: 1}
+    df2 = data.copy()
+    df2.attrs = {1: 2}
+    df = concat([df1, df2])
+    assert len(df.attrs) == 0
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        Series(data=[1, 2]),
+        DataFrame(
+            data={
+                "col1": [1, 2],
+            }
+        ),
+        DataFrame(dtype=float),
+        Series(dtype=float),
+    ],
+)
+def test_concat_retain_attrs(data):
+    # GH#41828
+    df1 = data.copy()
+    df1.attrs = {1: 1}
+    df2 = data.copy()
+    df2.attrs = {1: 1}
+    df = concat([df1, df2])
+    assert df.attrs[1] == 1
+
+
+@td.skip_array_manager_invalid_test
+@pytest.mark.parametrize("df_dtype", ["float64", "int64", "datetime64[ns]"])
+@pytest.mark.parametrize("empty_dtype", [None, "float64", "object"])
+def test_concat_ignore_empty_object_float(empty_dtype, df_dtype):
+    # https://github.com/pandas-dev/pandas/issues/45637
+    df = DataFrame({"foo": [1, 2], "bar": [1, 2]}, dtype=df_dtype)
+    empty = DataFrame(columns=["foo", "bar"], dtype=empty_dtype)
+
+    msg = "The behavior of DataFrame concatenation with empty or all-NA entries"
+    warn = None
+    if df_dtype == "datetime64[ns]" or (
+        df_dtype == "float64" and empty_dtype != "float64"
+    ):
+        warn = FutureWarning
+    with tm.assert_produces_warning(warn, match=msg):
+        result = concat([empty, df])
+    expected = df
+    if df_dtype == "int64":
+        # TODO what exact behaviour do we want for integer eventually?
+        if empty_dtype == "float64":
+            expected = df.astype("float64")
+        else:
+            expected = df.astype("object")
+    tm.assert_frame_equal(result, expected)
+
+
+@td.skip_array_manager_invalid_test
+@pytest.mark.parametrize("df_dtype", ["float64", "int64", "datetime64[ns]"])
+@pytest.mark.parametrize("empty_dtype", [None, "float64", "object"])
+def test_concat_ignore_all_na_object_float(empty_dtype, df_dtype):
+    df = DataFrame({"foo": [1, 2], "bar": [1, 2]}, dtype=df_dtype)
+    empty = DataFrame({"foo": [np.nan], "bar": [np.nan]}, dtype=empty_dtype)
+
+    if df_dtype == "int64":
+        # TODO what exact behaviour do we want for integer eventually?
+        if empty_dtype == "object":
+            df_dtype = "object"
+        else:
+            df_dtype = "float64"
+
+    msg = "The behavior of DataFrame concatenation with empty or all-NA entries"
+    warn = None
+    if empty_dtype != df_dtype and empty_dtype is not None:
+        warn = FutureWarning
+    elif df_dtype == "datetime64[ns]":
+        warn = FutureWarning
+
+    with tm.assert_produces_warning(warn, match=msg):
+        result = concat([empty, df], ignore_index=True)
+
+    expected = DataFrame({"foo": [np.nan, 1, 2], "bar": [np.nan, 1, 2]}, dtype=df_dtype)
+    tm.assert_frame_equal(result, expected)
+
+
+@td.skip_array_manager_invalid_test
+def test_concat_ignore_empty_from_reindex():
+    # https://github.com/pandas-dev/pandas/pull/43507#issuecomment-920375856
+    df1 = DataFrame({"a": [1], "b": [pd.Timestamp("2012-01-01")]})
+    df2 = DataFrame({"a": [2]})
+
+    aligned = df2.reindex(columns=df1.columns)
+
+    msg = "The behavior of DataFrame concatenation with empty or all-NA entries"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        result = concat([df1, aligned], ignore_index=True)
+    expected = df1 = DataFrame({"a": [1, 2], "b": [pd.Timestamp("2012-01-01"), pd.NaT]})
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_mismatched_keys_length():
+    # GH#43485
+    ser = Series(range(5))
+    sers = [ser + n for n in range(4)]
+    keys = ["A", "B", "C"]
+
+    msg = r"The behavior of pd.concat with len\(keys\) != len\(objs\) is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        concat(sers, keys=keys, axis=1)
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        concat(sers, keys=keys, axis=0)
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        concat((x for x in sers), keys=(y for y in keys), axis=1)
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        concat((x for x in sers), keys=(y for y in keys), axis=0)
+
+
+def test_concat_multiindex_with_category():
+    df1 = DataFrame(
+        {
+            "c1": Series(list("abc"), dtype="category"),
+            "c2": Series(list("eee"), dtype="category"),
+            "i2": Series([1, 2, 3]),
+        }
+    )
+    df1 = df1.set_index(["c1", "c2"])
+    df2 = DataFrame(
+        {
+            "c1": Series(list("abc"), dtype="category"),
+            "c2": Series(list("eee"), dtype="category"),
+            "i2": Series([4, 5, 6]),
+        }
+    )
+    df2 = df2.set_index(["c1", "c2"])
+    result = concat([df1, df2])
+    expected = DataFrame(
+        {
+            "c1": Series(list("abcabc"), dtype="category"),
+            "c2": Series(list("eeeeee"), dtype="category"),
+            "i2": Series([1, 2, 3, 4, 5, 6]),
+        }
+    )
+    expected = expected.set_index(["c1", "c2"])
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_ea_upcast():
+    # GH#54848
+    df1 = DataFrame(["a"], dtype="string")
+    df2 = DataFrame([1], dtype="Int64")
+    result = concat([df1, df2])
+    expected = DataFrame(["a", 1], index=[0, 0])
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_none_with_timezone_timestamp():
+    # GH#52093
+    df1 = DataFrame([{"A": None}])
+    df2 = DataFrame([{"A": pd.Timestamp("1990-12-20 00:00:00+00:00")}])
+    msg = "The behavior of DataFrame concatenation with empty or all-NA entries"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        result = concat([df1, df2], ignore_index=True)
+    expected = DataFrame({"A": [None, pd.Timestamp("1990-12-20 00:00:00+00:00")]})
+    tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_dataframe.py

@@ -0,0 +1,230 @@
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    Index,
+    Series,
+    concat,
+)
+import pandas._testing as tm
+
+
+class TestDataFrameConcat:
+    def test_concat_multiple_frames_dtypes(self):
+        # GH#2759
+        df1 = DataFrame(data=np.ones((10, 2)), columns=["foo", "bar"], dtype=np.float64)
+        df2 = DataFrame(data=np.ones((10, 2)), dtype=np.float32)
+        results = concat((df1, df2), axis=1).dtypes
+        expected = Series(
+            [np.dtype("float64")] * 2 + [np.dtype("float32")] * 2,
+            index=["foo", "bar", 0, 1],
+        )
+        tm.assert_series_equal(results, expected)
+
+    def test_concat_tuple_keys(self):
+        # GH#14438
+        df1 = DataFrame(np.ones((2, 2)), columns=list("AB"))
+        df2 = DataFrame(np.ones((3, 2)) * 2, columns=list("AB"))
+        results = concat((df1, df2), keys=[("bee", "bah"), ("bee", "boo")])
+        expected = DataFrame(
+            {
+                "A": {
+                    ("bee", "bah", 0): 1.0,
+                    ("bee", "bah", 1): 1.0,
+                    ("bee", "boo", 0): 2.0,
+                    ("bee", "boo", 1): 2.0,
+                    ("bee", "boo", 2): 2.0,
+                },
+                "B": {
+                    ("bee", "bah", 0): 1.0,
+                    ("bee", "bah", 1): 1.0,
+                    ("bee", "boo", 0): 2.0,
+                    ("bee", "boo", 1): 2.0,
+                    ("bee", "boo", 2): 2.0,
+                },
+            }
+        )
+        tm.assert_frame_equal(results, expected)
+
+    def test_concat_named_keys(self):
+        # GH#14252
+        df = DataFrame({"foo": [1, 2], "bar": [0.1, 0.2]})
+        index = Index(["a", "b"], name="baz")
+        concatted_named_from_keys = concat([df, df], keys=index)
+        expected_named = DataFrame(
+            {"foo": [1, 2, 1, 2], "bar": [0.1, 0.2, 0.1, 0.2]},
+            index=pd.MultiIndex.from_product((["a", "b"], [0, 1]), names=["baz", None]),
+        )
+        tm.assert_frame_equal(concatted_named_from_keys, expected_named)
+
+        index_no_name = Index(["a", "b"], name=None)
+        concatted_named_from_names = concat([df, df], keys=index_no_name, names=["baz"])
+        tm.assert_frame_equal(concatted_named_from_names, expected_named)
+
+        concatted_unnamed = concat([df, df], keys=index_no_name)
+        expected_unnamed = DataFrame(
+            {"foo": [1, 2, 1, 2], "bar": [0.1, 0.2, 0.1, 0.2]},
+            index=pd.MultiIndex.from_product((["a", "b"], [0, 1]), names=[None, None]),
+        )
+        tm.assert_frame_equal(concatted_unnamed, expected_unnamed)
+
+    def test_concat_axis_parameter(self):
+        # GH#14369
+        df1 = DataFrame({"A": [0.1, 0.2]}, index=range(2))
+        df2 = DataFrame({"A": [0.3, 0.4]}, index=range(2))
+
+        # Index/row/0 DataFrame
+        expected_index = DataFrame({"A": [0.1, 0.2, 0.3, 0.4]}, index=[0, 1, 0, 1])
+
+        concatted_index = concat([df1, df2], axis="index")
+        tm.assert_frame_equal(concatted_index, expected_index)
+
+        concatted_row = concat([df1, df2], axis="rows")
+        tm.assert_frame_equal(concatted_row, expected_index)
+
+        concatted_0 = concat([df1, df2], axis=0)
+        tm.assert_frame_equal(concatted_0, expected_index)
+
+        # Columns/1 DataFrame
+        expected_columns = DataFrame(
+            [[0.1, 0.3], [0.2, 0.4]], index=[0, 1], columns=["A", "A"]
+        )
+
+        concatted_columns = concat([df1, df2], axis="columns")
+        tm.assert_frame_equal(concatted_columns, expected_columns)
+
+        concatted_1 = concat([df1, df2], axis=1)
+        tm.assert_frame_equal(concatted_1, expected_columns)
+
+        series1 = Series([0.1, 0.2])
+        series2 = Series([0.3, 0.4])
+
+        # Index/row/0 Series
+        expected_index_series = Series([0.1, 0.2, 0.3, 0.4], index=[0, 1, 0, 1])
+
+        concatted_index_series = concat([series1, series2], axis="index")
+        tm.assert_series_equal(concatted_index_series, expected_index_series)
+
+        concatted_row_series = concat([series1, series2], axis="rows")
+        tm.assert_series_equal(concatted_row_series, expected_index_series)
+
+        concatted_0_series = concat([series1, series2], axis=0)
+        tm.assert_series_equal(concatted_0_series, expected_index_series)
+
+        # Columns/1 Series
+        expected_columns_series = DataFrame(
+            [[0.1, 0.3], [0.2, 0.4]], index=[0, 1], columns=[0, 1]
+        )
+
+        concatted_columns_series = concat([series1, series2], axis="columns")
+        tm.assert_frame_equal(concatted_columns_series, expected_columns_series)
+
+        concatted_1_series = concat([series1, series2], axis=1)
+        tm.assert_frame_equal(concatted_1_series, expected_columns_series)
+
+        # Testing ValueError
+        with pytest.raises(ValueError, match="No axis named"):
+            concat([series1, series2], axis="something")
+
+    def test_concat_numerical_names(self):
+        # GH#15262, GH#12223
+        df = DataFrame(
+            {"col": range(9)},
+            dtype="int32",
+            index=(
+                pd.MultiIndex.from_product(
+                    [["A0", "A1", "A2"], ["B0", "B1", "B2"]], names=[1, 2]
+                )
+            ),
+        )
+        result = concat((df.iloc[:2, :], df.iloc[-2:, :]))
+        expected = DataFrame(
+            {"col": [0, 1, 7, 8]},
+            dtype="int32",
+            index=pd.MultiIndex.from_tuples(
+                [("A0", "B0"), ("A0", "B1"), ("A2", "B1"), ("A2", "B2")], names=[1, 2]
+            ),
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_astype_dup_col(self):
+        # GH#23049
+        df = DataFrame([{"a": "b"}])
+        df = concat([df, df], axis=1)
+
+        result = df.astype("category")
+        expected = DataFrame(
+            np.array(["b", "b"]).reshape(1, 2), columns=["a", "a"]
+        ).astype("category")
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_dataframe_keys_bug(self, sort):
+        t1 = DataFrame(
+            {"value": Series([1, 2, 3], index=Index(["a", "b", "c"], name="id"))}
+        )
+        t2 = DataFrame({"value": Series([7, 8], index=Index(["a", "b"], name="id"))})
+
+        # it works
+        result = concat([t1, t2], axis=1, keys=["t1", "t2"], sort=sort)
+        assert list(result.columns) == [("t1", "value"), ("t2", "value")]
+
+    def test_concat_bool_with_int(self):
+        # GH#42092 we may want to change this to return object, but that
+        #  would need a deprecation
+        df1 = DataFrame(Series([True, False, True, True], dtype="bool"))
+        df2 = DataFrame(Series([1, 0, 1], dtype="int64"))
+
+        result = concat([df1, df2])
+        expected = concat([df1.astype("int64"), df2])
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_duplicates_in_index_with_keys(self):
+        # GH#42651
+        index = [1, 1, 3]
+        data = [1, 2, 3]
+
+        df = DataFrame(data=data, index=index)
+        result = concat([df], keys=["A"], names=["ID", "date"])
+        mi = pd.MultiIndex.from_product([["A"], index], names=["ID", "date"])
+        expected = DataFrame(data=data, index=mi)
+        tm.assert_frame_equal(result, expected)
+        tm.assert_index_equal(result.index.levels[1], Index([1, 3], name="date"))
+
+    @pytest.mark.parametrize("ignore_index", [True, False])
+    @pytest.mark.parametrize("order", ["C", "F"])
+    @pytest.mark.parametrize("axis", [0, 1])
+    def test_concat_copies(self, axis, order, ignore_index, using_copy_on_write):
+        # based on asv ConcatDataFrames
+        df = DataFrame(np.zeros((10, 5), dtype=np.float32, order=order))
+
+        res = concat([df] * 5, axis=axis, ignore_index=ignore_index, copy=True)
+
+        if not using_copy_on_write:
+            for arr in res._iter_column_arrays():
+                for arr2 in df._iter_column_arrays():
+                    assert not np.shares_memory(arr, arr2)
+
+    def test_outer_sort_columns(self):
+        # GH#47127
+        df1 = DataFrame({"A": [0], "B": [1], 0: 1})
+        df2 = DataFrame({"A": [100]})
+        result = concat([df1, df2], ignore_index=True, join="outer", sort=True)
+        expected = DataFrame({0: [1.0, np.nan], "A": [0, 100], "B": [1.0, np.nan]})
+        tm.assert_frame_equal(result, expected)
+
+    def test_inner_sort_columns(self):
+        # GH#47127
+        df1 = DataFrame({"A": [0], "B": [1], 0: 1})
+        df2 = DataFrame({"A": [100], 0: 2})
+        result = concat([df1, df2], ignore_index=True, join="inner", sort=True)
+        expected = DataFrame({0: [1, 2], "A": [0, 100]})
+        tm.assert_frame_equal(result, expected)
+
+    def test_sort_columns_one_df(self):
+        # GH#47127
+        df1 = DataFrame({"A": [100], 0: 2})
+        result = concat([df1], ignore_index=True, join="inner", sort=True)
+        expected = DataFrame({0: [2], "A": [100]})
+        tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_datetimes.py

@@ -0,0 +1,606 @@
+import datetime as dt
+from datetime import datetime
+
+import dateutil
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    DatetimeIndex,
+    Index,
+    MultiIndex,
+    Series,
+    Timestamp,
+    concat,
+    date_range,
+    to_timedelta,
+)
+import pandas._testing as tm
+
+
+class TestDatetimeConcat:
+    def test_concat_datetime64_block(self):
+        rng = date_range("1/1/2000", periods=10)
+
+        df = DataFrame({"time": rng})
+
+        result = concat([df, df])
+        assert (result.iloc[:10]["time"] == rng).all()
+        assert (result.iloc[10:]["time"] == rng).all()
+
+    def test_concat_datetime_datetime64_frame(self):
+        # GH#2624
+        rows = []
+        rows.append([datetime(2010, 1, 1), 1])
+        rows.append([datetime(2010, 1, 2), "hi"])
+
+        df2_obj = DataFrame.from_records(rows, columns=["date", "test"])
+
+        ind = date_range(start="2000/1/1", freq="D", periods=10)
+        df1 = DataFrame({"date": ind, "test": range(10)})
+
+        # it works!
+        concat([df1, df2_obj])
+
+    def test_concat_datetime_timezone(self):
+        # GH 18523
+        idx1 = date_range("2011-01-01", periods=3, freq="h", tz="Europe/Paris")
+        idx2 = date_range(start=idx1[0], end=idx1[-1], freq="h")
+        df1 = DataFrame({"a": [1, 2, 3]}, index=idx1)
+        df2 = DataFrame({"b": [1, 2, 3]}, index=idx2)
+        result = concat([df1, df2], axis=1)
+
+        exp_idx = DatetimeIndex(
+            [
+                "2011-01-01 00:00:00+01:00",
+                "2011-01-01 01:00:00+01:00",
+                "2011-01-01 02:00:00+01:00",
+            ],
+            dtype="M8[ns, Europe/Paris]",
+            freq="h",
+        )
+        expected = DataFrame(
+            [[1, 1], [2, 2], [3, 3]], index=exp_idx, columns=["a", "b"]
+        )
+
+        tm.assert_frame_equal(result, expected)
+
+        idx3 = date_range("2011-01-01", periods=3, freq="h", tz="Asia/Tokyo")
+        df3 = DataFrame({"b": [1, 2, 3]}, index=idx3)
+        result = concat([df1, df3], axis=1)
+
+        exp_idx = DatetimeIndex(
+            [
+                "2010-12-31 15:00:00+00:00",
+                "2010-12-31 16:00:00+00:00",
+                "2010-12-31 17:00:00+00:00",
+                "2010-12-31 23:00:00+00:00",
+                "2011-01-01 00:00:00+00:00",
+                "2011-01-01 01:00:00+00:00",
+            ]
+        ).as_unit("ns")
+
+        expected = DataFrame(
+            [
+                [np.nan, 1],
+                [np.nan, 2],
+                [np.nan, 3],
+                [1, np.nan],
+                [2, np.nan],
+                [3, np.nan],
+            ],
+            index=exp_idx,
+            columns=["a", "b"],
+        )
+
+        tm.assert_frame_equal(result, expected)
+
+        # GH 13783: Concat after resample
+        result = concat([df1.resample("h").mean(), df2.resample("h").mean()], sort=True)
+        expected = DataFrame(
+            {"a": [1, 2, 3] + [np.nan] * 3, "b": [np.nan] * 3 + [1, 2, 3]},
+            index=idx1.append(idx1),
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_datetimeindex_freq(self):
+        # GH 3232
+        # Monotonic index result
+        dr = date_range("01-Jan-2013", periods=100, freq="50ms", tz="UTC")
+        data = list(range(100))
+        expected = DataFrame(data, index=dr)
+        result = concat([expected[:50], expected[50:]])
+        tm.assert_frame_equal(result, expected)
+
+        # Non-monotonic index result
+        result = concat([expected[50:], expected[:50]])
+        expected = DataFrame(data[50:] + data[:50], index=dr[50:].append(dr[:50]))
+        expected.index._data.freq = None
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_multiindex_datetime_object_index(self):
+        # https://github.com/pandas-dev/pandas/issues/11058
+        idx = Index(
+            [dt.date(2013, 1, 1), dt.date(2014, 1, 1), dt.date(2015, 1, 1)],
+            dtype="object",
+        )
+
+        s = Series(
+            ["a", "b"],
+            index=MultiIndex.from_arrays(
+                [
+                    [1, 2],
+                    idx[:-1],
+                ],
+                names=["first", "second"],
+            ),
+        )
+        s2 = Series(
+            ["a", "b"],
+            index=MultiIndex.from_arrays(
+                [[1, 2], idx[::2]],
+                names=["first", "second"],
+            ),
+        )
+        mi = MultiIndex.from_arrays(
+            [[1, 2, 2], idx],
+            names=["first", "second"],
+        )
+        assert mi.levels[1].dtype == object
+
+        expected = DataFrame(
+            [["a", "a"], ["b", np.nan], [np.nan, "b"]],
+            index=mi,
+        )
+        result = concat([s, s2], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_NaT_series(self):
+        # GH 11693
+        # test for merging NaT series with datetime series.
+        x = Series(
+            date_range("20151124 08:00", "20151124 09:00", freq="1h", tz="US/Eastern")
+        )
+        y = Series(pd.NaT, index=[0, 1], dtype="datetime64[ns, US/Eastern]")
+        expected = Series([x[0], x[1], pd.NaT, pd.NaT])
+
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+        # all NaT with tz
+        expected = Series(pd.NaT, index=range(4), dtype="datetime64[ns, US/Eastern]")
+        result = concat([y, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_NaT_series2(self):
+        # without tz
+        x = Series(date_range("20151124 08:00", "20151124 09:00", freq="1h"))
+        y = Series(date_range("20151124 10:00", "20151124 11:00", freq="1h"))
+        y[:] = pd.NaT
+        expected = Series([x[0], x[1], pd.NaT, pd.NaT])
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+        # all NaT without tz
+        x[:] = pd.NaT
+        expected = Series(pd.NaT, index=range(4), dtype="datetime64[ns]")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize("tz", [None, "UTC"])
+    def test_concat_NaT_dataframes(self, tz):
+        # GH 12396
+
+        dti = DatetimeIndex([pd.NaT, pd.NaT], tz=tz)
+        first = DataFrame({0: dti})
+        second = DataFrame(
+            [[Timestamp("2015/01/01", tz=tz)], [Timestamp("2016/01/01", tz=tz)]],
+            index=[2, 3],
+        )
+        expected = DataFrame(
+            [
+                pd.NaT,
+                pd.NaT,
+                Timestamp("2015/01/01", tz=tz),
+                Timestamp("2016/01/01", tz=tz),
+            ]
+        )
+
+        result = concat([first, second], axis=0)
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("tz1", [None, "UTC"])
+    @pytest.mark.parametrize("tz2", [None, "UTC"])
+    @pytest.mark.parametrize("item", [pd.NaT, Timestamp("20150101")])
+    def test_concat_NaT_dataframes_all_NaT_axis_0(
+        self, tz1, tz2, item, using_array_manager
+    ):
+        # GH 12396
+
+        # tz-naive
+        first = DataFrame([[pd.NaT], [pd.NaT]]).apply(lambda x: x.dt.tz_localize(tz1))
+        second = DataFrame([item]).apply(lambda x: x.dt.tz_localize(tz2))
+
+        result = concat([first, second], axis=0)
+        expected = DataFrame(Series([pd.NaT, pd.NaT, item], index=[0, 1, 0]))
+        expected = expected.apply(lambda x: x.dt.tz_localize(tz2))
+        if tz1 != tz2:
+            expected = expected.astype(object)
+            if item is pd.NaT and not using_array_manager:
+                # GH#18463
+                # TODO: setting nan here is to keep the test passing as we
+                #  make assert_frame_equal stricter, but is nan really the
+                #  ideal behavior here?
+                if tz1 is not None:
+                    expected.iloc[-1, 0] = np.nan
+                else:
+                    expected.iloc[:-1, 0] = np.nan
+
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("tz1", [None, "UTC"])
+    @pytest.mark.parametrize("tz2", [None, "UTC"])
+    def test_concat_NaT_dataframes_all_NaT_axis_1(self, tz1, tz2):
+        # GH 12396
+
+        first = DataFrame(Series([pd.NaT, pd.NaT]).dt.tz_localize(tz1))
+        second = DataFrame(Series([pd.NaT]).dt.tz_localize(tz2), columns=[1])
+        expected = DataFrame(
+            {
+                0: Series([pd.NaT, pd.NaT]).dt.tz_localize(tz1),
+                1: Series([pd.NaT, pd.NaT]).dt.tz_localize(tz2),
+            }
+        )
+        result = concat([first, second], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("tz1", [None, "UTC"])
+    @pytest.mark.parametrize("tz2", [None, "UTC"])
+    def test_concat_NaT_series_dataframe_all_NaT(self, tz1, tz2):
+        # GH 12396
+
+        # tz-naive
+        first = Series([pd.NaT, pd.NaT]).dt.tz_localize(tz1)
+        second = DataFrame(
+            [
+                [Timestamp("2015/01/01", tz=tz2)],
+                [Timestamp("2016/01/01", tz=tz2)],
+            ],
+            index=[2, 3],
+        )
+
+        expected = DataFrame(
+            [
+                pd.NaT,
+                pd.NaT,
+                Timestamp("2015/01/01", tz=tz2),
+                Timestamp("2016/01/01", tz=tz2),
+            ]
+        )
+        if tz1 != tz2:
+            expected = expected.astype(object)
+
+        result = concat([first, second])
+        tm.assert_frame_equal(result, expected)
+
+
+class TestTimezoneConcat:
+    def test_concat_tz_series(self):
+        # gh-11755: tz and no tz
+        x = Series(date_range("20151124 08:00", "20151124 09:00", freq="1h", tz="UTC"))
+        y = Series(date_range("2012-01-01", "2012-01-02"))
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="object")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_tz_series2(self):
+        # gh-11887: concat tz and object
+        x = Series(date_range("20151124 08:00", "20151124 09:00", freq="1h", tz="UTC"))
+        y = Series(["a", "b"])
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="object")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_tz_series3(self, unit, unit2):
+        # see gh-12217 and gh-12306
+        # Concatenating two UTC times
+        first = DataFrame([[datetime(2016, 1, 1)]], dtype=f"M8[{unit}]")
+        first[0] = first[0].dt.tz_localize("UTC")
+
+        second = DataFrame([[datetime(2016, 1, 2)]], dtype=f"M8[{unit2}]")
+        second[0] = second[0].dt.tz_localize("UTC")
+
+        result = concat([first, second])
+        exp_unit = tm.get_finest_unit(unit, unit2)
+        assert result[0].dtype == f"datetime64[{exp_unit}, UTC]"
+
+    def test_concat_tz_series4(self, unit, unit2):
+        # Concatenating two London times
+        first = DataFrame([[datetime(2016, 1, 1)]], dtype=f"M8[{unit}]")
+        first[0] = first[0].dt.tz_localize("Europe/London")
+
+        second = DataFrame([[datetime(2016, 1, 2)]], dtype=f"M8[{unit2}]")
+        second[0] = second[0].dt.tz_localize("Europe/London")
+
+        result = concat([first, second])
+        exp_unit = tm.get_finest_unit(unit, unit2)
+        assert result[0].dtype == f"datetime64[{exp_unit}, Europe/London]"
+
+    def test_concat_tz_series5(self, unit, unit2):
+        # Concatenating 2+1 London times
+        first = DataFrame(
+            [[datetime(2016, 1, 1)], [datetime(2016, 1, 2)]], dtype=f"M8[{unit}]"
+        )
+        first[0] = first[0].dt.tz_localize("Europe/London")
+
+        second = DataFrame([[datetime(2016, 1, 3)]], dtype=f"M8[{unit2}]")
+        second[0] = second[0].dt.tz_localize("Europe/London")
+
+        result = concat([first, second])
+        exp_unit = tm.get_finest_unit(unit, unit2)
+        assert result[0].dtype == f"datetime64[{exp_unit}, Europe/London]"
+
+    def test_concat_tz_series6(self, unit, unit2):
+        # Concatenating 1+2 London times
+        first = DataFrame([[datetime(2016, 1, 1)]], dtype=f"M8[{unit}]")
+        first[0] = first[0].dt.tz_localize("Europe/London")
+
+        second = DataFrame(
+            [[datetime(2016, 1, 2)], [datetime(2016, 1, 3)]], dtype=f"M8[{unit2}]"
+        )
+        second[0] = second[0].dt.tz_localize("Europe/London")
+
+        result = concat([first, second])
+        exp_unit = tm.get_finest_unit(unit, unit2)
+        assert result[0].dtype == f"datetime64[{exp_unit}, Europe/London]"
+
+    def test_concat_tz_series_tzlocal(self):
+        # see gh-13583
+        x = [
+            Timestamp("2011-01-01", tz=dateutil.tz.tzlocal()),
+            Timestamp("2011-02-01", tz=dateutil.tz.tzlocal()),
+        ]
+        y = [
+            Timestamp("2012-01-01", tz=dateutil.tz.tzlocal()),
+            Timestamp("2012-02-01", tz=dateutil.tz.tzlocal()),
+        ]
+
+        result = concat([Series(x), Series(y)], ignore_index=True)
+        tm.assert_series_equal(result, Series(x + y))
+        assert result.dtype == "datetime64[ns, tzlocal()]"
+
+    def test_concat_tz_series_with_datetimelike(self):
+        # see gh-12620: tz and timedelta
+        x = [
+            Timestamp("2011-01-01", tz="US/Eastern"),
+            Timestamp("2011-02-01", tz="US/Eastern"),
+        ]
+        y = [pd.Timedelta("1 day"), pd.Timedelta("2 day")]
+        result = concat([Series(x), Series(y)], ignore_index=True)
+        tm.assert_series_equal(result, Series(x + y, dtype="object"))
+
+        # tz and period
+        y = [pd.Period("2011-03", freq="M"), pd.Period("2011-04", freq="M")]
+        result = concat([Series(x), Series(y)], ignore_index=True)
+        tm.assert_series_equal(result, Series(x + y, dtype="object"))
+
+    def test_concat_tz_frame(self):
+        df2 = DataFrame(
+            {
+                "A": Timestamp("20130102", tz="US/Eastern"),
+                "B": Timestamp("20130603", tz="CET"),
+            },
+            index=range(5),
+        )
+
+        # concat
+        df3 = concat([df2.A.to_frame(), df2.B.to_frame()], axis=1)
+        tm.assert_frame_equal(df2, df3)
+
+    def test_concat_multiple_tzs(self):
+        # GH#12467
+        # combining datetime tz-aware and naive DataFrames
+        ts1 = Timestamp("2015-01-01", tz=None)
+        ts2 = Timestamp("2015-01-01", tz="UTC")
+        ts3 = Timestamp("2015-01-01", tz="EST")
+
+        df1 = DataFrame({"time": [ts1]})
+        df2 = DataFrame({"time": [ts2]})
+        df3 = DataFrame({"time": [ts3]})
+
+        results = concat([df1, df2]).reset_index(drop=True)
+        expected = DataFrame({"time": [ts1, ts2]}, dtype=object)
+        tm.assert_frame_equal(results, expected)
+
+        results = concat([df1, df3]).reset_index(drop=True)
+        expected = DataFrame({"time": [ts1, ts3]}, dtype=object)
+        tm.assert_frame_equal(results, expected)
+
+        results = concat([df2, df3]).reset_index(drop=True)
+        expected = DataFrame({"time": [ts2, ts3]})
+        tm.assert_frame_equal(results, expected)
+
+    def test_concat_multiindex_with_tz(self):
+        # GH 6606
+        df = DataFrame(
+            {
+                "dt": DatetimeIndex(
+                    [
+                        datetime(2014, 1, 1),
+                        datetime(2014, 1, 2),
+                        datetime(2014, 1, 3),
+                    ],
+                    dtype="M8[ns, US/Pacific]",
+                ),
+                "b": ["A", "B", "C"],
+                "c": [1, 2, 3],
+                "d": [4, 5, 6],
+            }
+        )
+        df = df.set_index(["dt", "b"])
+
+        exp_idx1 = DatetimeIndex(
+            ["2014-01-01", "2014-01-02", "2014-01-03"] * 2,
+            dtype="M8[ns, US/Pacific]",
+            name="dt",
+        )
+        exp_idx2 = Index(["A", "B", "C"] * 2, name="b")
+        exp_idx = MultiIndex.from_arrays([exp_idx1, exp_idx2])
+        expected = DataFrame(
+            {"c": [1, 2, 3] * 2, "d": [4, 5, 6] * 2}, index=exp_idx, columns=["c", "d"]
+        )
+
+        result = concat([df, df])
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_tz_not_aligned(self):
+        # GH#22796
+        ts = pd.to_datetime([1, 2]).tz_localize("UTC")
+        a = DataFrame({"A": ts})
+        b = DataFrame({"A": ts, "B": ts})
+        result = concat([a, b], sort=True, ignore_index=True)
+        expected = DataFrame(
+            {"A": list(ts) + list(ts), "B": [pd.NaT, pd.NaT] + list(ts)}
+        )
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "t1",
+        [
+            "2015-01-01",
+            pytest.param(
+                pd.NaT,
+                marks=pytest.mark.xfail(
+                    reason="GH23037 incorrect dtype when concatenating"
+                ),
+            ),
+        ],
+    )
+    def test_concat_tz_NaT(self, t1):
+        # GH#22796
+        # Concatenating tz-aware multicolumn DataFrames
+        ts1 = Timestamp(t1, tz="UTC")
+        ts2 = Timestamp("2015-01-01", tz="UTC")
+        ts3 = Timestamp("2015-01-01", tz="UTC")
+
+        df1 = DataFrame([[ts1, ts2]])
+        df2 = DataFrame([[ts3]])
+
+        result = concat([df1, df2])
+        expected = DataFrame([[ts1, ts2], [ts3, pd.NaT]], index=[0, 0])
+
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_tz_with_empty(self):
+        # GH 9188
+        result = concat(
+            [DataFrame(date_range("2000", periods=1, tz="UTC")), DataFrame()]
+        )
+        expected = DataFrame(date_range("2000", periods=1, tz="UTC"))
+        tm.assert_frame_equal(result, expected)
+
+
+class TestPeriodConcat:
+    def test_concat_period_series(self):
+        x = Series(pd.PeriodIndex(["2015-11-01", "2015-12-01"], freq="D"))
+        y = Series(pd.PeriodIndex(["2015-10-01", "2016-01-01"], freq="D"))
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="Period[D]")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_period_multiple_freq_series(self):
+        x = Series(pd.PeriodIndex(["2015-11-01", "2015-12-01"], freq="D"))
+        y = Series(pd.PeriodIndex(["2015-10-01", "2016-01-01"], freq="M"))
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="object")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+        assert result.dtype == "object"
+
+    def test_concat_period_other_series(self):
+        x = Series(pd.PeriodIndex(["2015-11-01", "2015-12-01"], freq="D"))
+        y = Series(pd.PeriodIndex(["2015-11-01", "2015-12-01"], freq="M"))
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="object")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+        assert result.dtype == "object"
+
+    def test_concat_period_other_series2(self):
+        # non-period
+        x = Series(pd.PeriodIndex(["2015-11-01", "2015-12-01"], freq="D"))
+        y = Series(DatetimeIndex(["2015-11-01", "2015-12-01"]))
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="object")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+        assert result.dtype == "object"
+
+    def test_concat_period_other_series3(self):
+        x = Series(pd.PeriodIndex(["2015-11-01", "2015-12-01"], freq="D"))
+        y = Series(["A", "B"])
+        expected = Series([x[0], x[1], y[0], y[1]], dtype="object")
+        result = concat([x, y], ignore_index=True)
+        tm.assert_series_equal(result, expected)
+        assert result.dtype == "object"
+
+
+def test_concat_timedelta64_block():
+    rng = to_timedelta(np.arange(10), unit="s")
+
+    df = DataFrame({"time": rng})
+
+    result = concat([df, df])
+    tm.assert_frame_equal(result.iloc[:10], df)
+    tm.assert_frame_equal(result.iloc[10:], df)
+
+
+def test_concat_multiindex_datetime_nat():
+    # GH#44900
+    left = DataFrame({"a": 1}, index=MultiIndex.from_tuples([(1, pd.NaT)]))
+    right = DataFrame(
+        {"b": 2}, index=MultiIndex.from_tuples([(1, pd.NaT), (2, pd.NaT)])
+    )
+    result = concat([left, right], axis="columns")
+    expected = DataFrame(
+        {"a": [1.0, np.nan], "b": 2}, MultiIndex.from_tuples([(1, pd.NaT), (2, pd.NaT)])
+    )
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_float_datetime64(using_array_manager):
+    # GH#32934
+    df_time = DataFrame({"A": pd.array(["2000"], dtype="datetime64[ns]")})
+    df_float = DataFrame({"A": pd.array([1.0], dtype="float64")})
+
+    expected = DataFrame(
+        {
+            "A": [
+                pd.array(["2000"], dtype="datetime64[ns]")[0],
+                pd.array([1.0], dtype="float64")[0],
+            ]
+        },
+        index=[0, 0],
+    )
+    result = concat([df_time, df_float])
+    tm.assert_frame_equal(result, expected)
+
+    expected = DataFrame({"A": pd.array([], dtype="object")})
+    result = concat([df_time.iloc[:0], df_float.iloc[:0]])
+    tm.assert_frame_equal(result, expected)
+
+    expected = DataFrame({"A": pd.array([1.0], dtype="object")})
+    result = concat([df_time.iloc[:0], df_float])
+    tm.assert_frame_equal(result, expected)
+
+    if not using_array_manager:
+        expected = DataFrame({"A": pd.array(["2000"], dtype="datetime64[ns]")})
+        msg = "The behavior of DataFrame concatenation with empty or all-NA entries"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = concat([df_time, df_float.iloc[:0]])
+        tm.assert_frame_equal(result, expected)
+    else:
+        expected = DataFrame({"A": pd.array(["2000"], dtype="datetime64[ns]")}).astype(
+            {"A": "object"}
+        )
+        result = concat([df_time, df_float.iloc[:0]])
+        tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_empty.py

@@ -0,0 +1,295 @@
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    RangeIndex,
+    Series,
+    concat,
+    date_range,
+)
+import pandas._testing as tm
+
+
+class TestEmptyConcat:
+    def test_handle_empty_objects(self, sort, using_infer_string):
+        df = DataFrame(
+            np.random.default_rng(2).standard_normal((10, 4)), columns=list("abcd")
+        )
+
+        dfcopy = df[:5].copy()
+        dfcopy["foo"] = "bar"
+        empty = df[5:5]
+
+        frames = [dfcopy, empty, empty, df[5:]]
+        concatted = concat(frames, axis=0, sort=sort)
+
+        expected = df.reindex(columns=["a", "b", "c", "d", "foo"])
+        expected["foo"] = expected["foo"].astype(
+            object if not using_infer_string else "string[pyarrow_numpy]"
+        )
+        expected.loc[0:4, "foo"] = "bar"
+
+        tm.assert_frame_equal(concatted, expected)
+
+        # empty as first element with time series
+        # GH3259
+        df = DataFrame(
+            {"A": range(10000)}, index=date_range("20130101", periods=10000, freq="s")
+        )
+        empty = DataFrame()
+        result = concat([df, empty], axis=1)
+        tm.assert_frame_equal(result, df)
+        result = concat([empty, df], axis=1)
+        tm.assert_frame_equal(result, df)
+
+        result = concat([df, empty])
+        tm.assert_frame_equal(result, df)
+        result = concat([empty, df])
+        tm.assert_frame_equal(result, df)
+
+    def test_concat_empty_series(self):
+        # GH 11082
+        s1 = Series([1, 2, 3], name="x")
+        s2 = Series(name="y", dtype="float64")
+        res = concat([s1, s2], axis=1)
+        exp = DataFrame(
+            {"x": [1, 2, 3], "y": [np.nan, np.nan, np.nan]},
+            index=RangeIndex(3),
+        )
+        tm.assert_frame_equal(res, exp)
+
+        s1 = Series([1, 2, 3], name="x")
+        s2 = Series(name="y", dtype="float64")
+        msg = "The behavior of array concatenation with empty entries is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            res = concat([s1, s2], axis=0)
+        # name will be reset
+        exp = Series([1, 2, 3])
+        tm.assert_series_equal(res, exp)
+
+        # empty Series with no name
+        s1 = Series([1, 2, 3], name="x")
+        s2 = Series(name=None, dtype="float64")
+        res = concat([s1, s2], axis=1)
+        exp = DataFrame(
+            {"x": [1, 2, 3], 0: [np.nan, np.nan, np.nan]},
+            columns=["x", 0],
+            index=RangeIndex(3),
+        )
+        tm.assert_frame_equal(res, exp)
+
+    @pytest.mark.parametrize("tz", [None, "UTC"])
+    @pytest.mark.parametrize("values", [[], [1, 2, 3]])
+    def test_concat_empty_series_timelike(self, tz, values):
+        # GH 18447
+
+        first = Series([], dtype="M8[ns]").dt.tz_localize(tz)
+        dtype = None if values else np.float64
+        second = Series(values, dtype=dtype)
+
+        expected = DataFrame(
+            {
+                0: Series([pd.NaT] * len(values), dtype="M8[ns]").dt.tz_localize(tz),
+                1: values,
+            }
+        )
+        result = concat([first, second], axis=1)
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "left,right,expected",
+        [
+            # booleans
+            (np.bool_, np.int32, np.object_),  # changed from int32 in 2.0 GH#39817
+            (np.bool_, np.float32, np.object_),
+            # datetime-like
+            ("m8[ns]", np.bool_, np.object_),
+            ("m8[ns]", np.int64, np.object_),
+            ("M8[ns]", np.bool_, np.object_),
+            ("M8[ns]", np.int64, np.object_),
+            # categorical
+            ("category", "category", "category"),
+            ("category", "object", "object"),
+        ],
+    )
+    def test_concat_empty_series_dtypes(self, left, right, expected):
+        # GH#39817, GH#45101
+        result = concat([Series(dtype=left), Series(dtype=right)])
+        assert result.dtype == expected
+
+    @pytest.mark.parametrize(
+        "dtype", ["float64", "int8", "uint8", "bool", "m8[ns]", "M8[ns]"]
+    )
+    def test_concat_empty_series_dtypes_match_roundtrips(self, dtype):
+        dtype = np.dtype(dtype)
+
+        result = concat([Series(dtype=dtype)])
+        assert result.dtype == dtype
+
+        result = concat([Series(dtype=dtype), Series(dtype=dtype)])
+        assert result.dtype == dtype
+
+    @pytest.mark.parametrize("dtype", ["float64", "int8", "uint8", "m8[ns]", "M8[ns]"])
+    @pytest.mark.parametrize(
+        "dtype2",
+        ["float64", "int8", "uint8", "m8[ns]", "M8[ns]"],
+    )
+    def test_concat_empty_series_dtypes_roundtrips(self, dtype, dtype2):
+        # round-tripping with self & like self
+        if dtype == dtype2:
+            pytest.skip("same dtype is not applicable for test")
+
+        def int_result_type(dtype, dtype2):
+            typs = {dtype.kind, dtype2.kind}
+            if not len(typs - {"i", "u", "b"}) and (
+                dtype.kind == "i" or dtype2.kind == "i"
+            ):
+                return "i"
+            elif not len(typs - {"u", "b"}) and (
+                dtype.kind == "u" or dtype2.kind == "u"
+            ):
+                return "u"
+            return None
+
+        def float_result_type(dtype, dtype2):
+            typs = {dtype.kind, dtype2.kind}
+            if not len(typs - {"f", "i", "u"}) and (
+                dtype.kind == "f" or dtype2.kind == "f"
+            ):
+                return "f"
+            return None
+
+        def get_result_type(dtype, dtype2):
+            result = float_result_type(dtype, dtype2)
+            if result is not None:
+                return result
+            result = int_result_type(dtype, dtype2)
+            if result is not None:
+                return result
+            return "O"
+
+        dtype = np.dtype(dtype)
+        dtype2 = np.dtype(dtype2)
+        expected = get_result_type(dtype, dtype2)
+        result = concat([Series(dtype=dtype), Series(dtype=dtype2)]).dtype
+        assert result.kind == expected
+
+    def test_concat_empty_series_dtypes_triple(self):
+        assert (
+            concat(
+                [Series(dtype="M8[ns]"), Series(dtype=np.bool_), Series(dtype=np.int64)]
+            ).dtype
+            == np.object_
+        )
+
+    def test_concat_empty_series_dtype_category_with_array(self):
+        # GH#18515
+        assert (
+            concat(
+                [Series(np.array([]), dtype="category"), Series(dtype="float64")]
+            ).dtype
+            == "float64"
+        )
+
+    def test_concat_empty_series_dtypes_sparse(self):
+        result = concat(
+            [
+                Series(dtype="float64").astype("Sparse"),
+                Series(dtype="float64").astype("Sparse"),
+            ]
+        )
+        assert result.dtype == "Sparse[float64]"
+
+        result = concat(
+            [Series(dtype="float64").astype("Sparse"), Series(dtype="float64")]
+        )
+        expected = pd.SparseDtype(np.float64)
+        assert result.dtype == expected
+
+        result = concat(
+            [Series(dtype="float64").astype("Sparse"), Series(dtype="object")]
+        )
+        expected = pd.SparseDtype("object")
+        assert result.dtype == expected
+
+    def test_concat_empty_df_object_dtype(self):
+        # GH 9149
+        df_1 = DataFrame({"Row": [0, 1, 1], "EmptyCol": np.nan, "NumberCol": [1, 2, 3]})
+        df_2 = DataFrame(columns=df_1.columns)
+        result = concat([df_1, df_2], axis=0)
+        expected = df_1.astype(object)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_empty_dataframe_dtypes(self):
+        df = DataFrame(columns=list("abc"))
+        df["a"] = df["a"].astype(np.bool_)
+        df["b"] = df["b"].astype(np.int32)
+        df["c"] = df["c"].astype(np.float64)
+
+        result = concat([df, df])
+        assert result["a"].dtype == np.bool_
+        assert result["b"].dtype == np.int32
+        assert result["c"].dtype == np.float64
+
+        result = concat([df, df.astype(np.float64)])
+        assert result["a"].dtype == np.object_
+        assert result["b"].dtype == np.float64
+        assert result["c"].dtype == np.float64
+
+    def test_concat_inner_join_empty(self):
+        # GH 15328
+        df_empty = DataFrame()
+        df_a = DataFrame({"a": [1, 2]}, index=[0, 1], dtype="int64")
+        df_expected = DataFrame({"a": []}, index=RangeIndex(0), dtype="int64")
+
+        result = concat([df_a, df_empty], axis=1, join="inner")
+        tm.assert_frame_equal(result, df_expected)
+
+        result = concat([df_a, df_empty], axis=1, join="outer")
+        tm.assert_frame_equal(result, df_a)
+
+    def test_empty_dtype_coerce(self):
+        # xref to #12411
+        # xref to #12045
+        # xref to #11594
+        # see below
+
+        # 10571
+        df1 = DataFrame(data=[[1, None], [2, None]], columns=["a", "b"])
+        df2 = DataFrame(data=[[3, None], [4, None]], columns=["a", "b"])
+        result = concat([df1, df2])
+        expected = df1.dtypes
+        tm.assert_series_equal(result.dtypes, expected)
+
+    def test_concat_empty_dataframe(self):
+        # 39037
+        df1 = DataFrame(columns=["a", "b"])
+        df2 = DataFrame(columns=["b", "c"])
+        result = concat([df1, df2, df1])
+        expected = DataFrame(columns=["a", "b", "c"])
+        tm.assert_frame_equal(result, expected)
+
+        df3 = DataFrame(columns=["a", "b"])
+        df4 = DataFrame(columns=["b"])
+        result = concat([df3, df4])
+        expected = DataFrame(columns=["a", "b"])
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_empty_dataframe_different_dtypes(self, using_infer_string):
+        # 39037
+        df1 = DataFrame({"a": [1, 2, 3], "b": ["a", "b", "c"]})
+        df2 = DataFrame({"a": [1, 2, 3]})
+
+        result = concat([df1[:0], df2[:0]])
+        assert result["a"].dtype == np.int64
+        assert result["b"].dtype == np.object_ if not using_infer_string else "string"
+
+    def test_concat_to_empty_ea(self):
+        """48510 `concat` to an empty EA should maintain type EA dtype."""
+        df_empty = DataFrame({"a": pd.array([], dtype=pd.Int64Dtype())})
+        df_new = DataFrame({"a": pd.array([1, 2, 3], dtype=pd.Int64Dtype())})
+        expected = df_new.copy()
+        result = concat([df_empty, df_new])
+        tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_index.py

@@ -0,0 +1,472 @@
+from copy import deepcopy
+
+import numpy as np
+import pytest
+
+from pandas.errors import PerformanceWarning
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    Index,
+    MultiIndex,
+    Series,
+    concat,
+)
+import pandas._testing as tm
+
+
+class TestIndexConcat:
+    def test_concat_ignore_index(self, sort):
+        frame1 = DataFrame(
+            {"test1": ["a", "b", "c"], "test2": [1, 2, 3], "test3": [4.5, 3.2, 1.2]}
+        )
+        frame2 = DataFrame({"test3": [5.2, 2.2, 4.3]})
+        frame1.index = Index(["x", "y", "z"])
+        frame2.index = Index(["x", "y", "q"])
+
+        v1 = concat([frame1, frame2], axis=1, ignore_index=True, sort=sort)
+
+        nan = np.nan
+        expected = DataFrame(
+            [
+                [nan, nan, nan, 4.3],
+                ["a", 1, 4.5, 5.2],
+                ["b", 2, 3.2, 2.2],
+                ["c", 3, 1.2, nan],
+            ],
+            index=Index(["q", "x", "y", "z"]),
+        )
+        if not sort:
+            expected = expected.loc[["x", "y", "z", "q"]]
+
+        tm.assert_frame_equal(v1, expected)
+
+    @pytest.mark.parametrize(
+        "name_in1,name_in2,name_in3,name_out",
+        [
+            ("idx", "idx", "idx", "idx"),
+            ("idx", "idx", None, None),
+            ("idx", None, None, None),
+            ("idx1", "idx2", None, None),
+            ("idx1", "idx1", "idx2", None),
+            ("idx1", "idx2", "idx3", None),
+            (None, None, None, None),
+        ],
+    )
+    def test_concat_same_index_names(self, name_in1, name_in2, name_in3, name_out):
+        # GH13475
+        indices = [
+            Index(["a", "b", "c"], name=name_in1),
+            Index(["b", "c", "d"], name=name_in2),
+            Index(["c", "d", "e"], name=name_in3),
+        ]
+        frames = [
+            DataFrame({c: [0, 1, 2]}, index=i) for i, c in zip(indices, ["x", "y", "z"])
+        ]
+        result = concat(frames, axis=1)
+
+        exp_ind = Index(["a", "b", "c", "d", "e"], name=name_out)
+        expected = DataFrame(
+            {
+                "x": [0, 1, 2, np.nan, np.nan],
+                "y": [np.nan, 0, 1, 2, np.nan],
+                "z": [np.nan, np.nan, 0, 1, 2],
+            },
+            index=exp_ind,
+        )
+
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_rename_index(self):
+        a = DataFrame(
+            np.random.default_rng(2).random((3, 3)),
+            columns=list("ABC"),
+            index=Index(list("abc"), name="index_a"),
+        )
+        b = DataFrame(
+            np.random.default_rng(2).random((3, 3)),
+            columns=list("ABC"),
+            index=Index(list("abc"), name="index_b"),
+        )
+
+        result = concat([a, b], keys=["key0", "key1"], names=["lvl0", "lvl1"])
+
+        exp = concat([a, b], keys=["key0", "key1"], names=["lvl0"])
+        names = list(exp.index.names)
+        names[1] = "lvl1"
+        exp.index.set_names(names, inplace=True)
+
+        tm.assert_frame_equal(result, exp)
+        assert result.index.names == exp.index.names
+
+    def test_concat_copy_index_series(self, axis, using_copy_on_write):
+        # GH 29879
+        ser = Series([1, 2])
+        comb = concat([ser, ser], axis=axis, copy=True)
+        if not using_copy_on_write or axis in [0, "index"]:
+            assert comb.index is not ser.index
+        else:
+            assert comb.index is ser.index
+
+    def test_concat_copy_index_frame(self, axis, using_copy_on_write):
+        # GH 29879
+        df = DataFrame([[1, 2], [3, 4]], columns=["a", "b"])
+        comb = concat([df, df], axis=axis, copy=True)
+        if not using_copy_on_write:
+            assert not comb.index.is_(df.index)
+            assert not comb.columns.is_(df.columns)
+        elif axis in [0, "index"]:
+            assert not comb.index.is_(df.index)
+            assert comb.columns.is_(df.columns)
+        elif axis in [1, "columns"]:
+            assert comb.index.is_(df.index)
+            assert not comb.columns.is_(df.columns)
+
+    def test_default_index(self):
+        # is_series and ignore_index
+        s1 = Series([1, 2, 3], name="x")
+        s2 = Series([4, 5, 6], name="y")
+        res = concat([s1, s2], axis=1, ignore_index=True)
+        assert isinstance(res.columns, pd.RangeIndex)
+        exp = DataFrame([[1, 4], [2, 5], [3, 6]])
+        # use check_index_type=True to check the result have
+        # RangeIndex (default index)
+        tm.assert_frame_equal(res, exp, check_index_type=True, check_column_type=True)
+
+        # is_series and all inputs have no names
+        s1 = Series([1, 2, 3])
+        s2 = Series([4, 5, 6])
+        res = concat([s1, s2], axis=1, ignore_index=False)
+        assert isinstance(res.columns, pd.RangeIndex)
+        exp = DataFrame([[1, 4], [2, 5], [3, 6]])
+        exp.columns = pd.RangeIndex(2)
+        tm.assert_frame_equal(res, exp, check_index_type=True, check_column_type=True)
+
+        # is_dataframe and ignore_index
+        df1 = DataFrame({"A": [1, 2], "B": [5, 6]})
+        df2 = DataFrame({"A": [3, 4], "B": [7, 8]})
+
+        res = concat([df1, df2], axis=0, ignore_index=True)
+        exp = DataFrame([[1, 5], [2, 6], [3, 7], [4, 8]], columns=["A", "B"])
+        tm.assert_frame_equal(res, exp, check_index_type=True, check_column_type=True)
+
+        res = concat([df1, df2], axis=1, ignore_index=True)
+        exp = DataFrame([[1, 5, 3, 7], [2, 6, 4, 8]])
+        tm.assert_frame_equal(res, exp, check_index_type=True, check_column_type=True)
+
+    def test_dups_index(self):
+        # GH 4771
+
+        # single dtypes
+        df = DataFrame(
+            np.random.default_rng(2).integers(0, 10, size=40).reshape(10, 4),
+            columns=["A", "A", "C", "C"],
+        )
+
+        result = concat([df, df], axis=1)
+        tm.assert_frame_equal(result.iloc[:, :4], df)
+        tm.assert_frame_equal(result.iloc[:, 4:], df)
+
+        result = concat([df, df], axis=0)
+        tm.assert_frame_equal(result.iloc[:10], df)
+        tm.assert_frame_equal(result.iloc[10:], df)
+
+        # multi dtypes
+        df = concat(
+            [
+                DataFrame(
+                    np.random.default_rng(2).standard_normal((10, 4)),
+                    columns=["A", "A", "B", "B"],
+                ),
+                DataFrame(
+                    np.random.default_rng(2).integers(0, 10, size=20).reshape(10, 2),
+                    columns=["A", "C"],
+                ),
+            ],
+            axis=1,
+        )
+
+        result = concat([df, df], axis=1)
+        tm.assert_frame_equal(result.iloc[:, :6], df)
+        tm.assert_frame_equal(result.iloc[:, 6:], df)
+
+        result = concat([df, df], axis=0)
+        tm.assert_frame_equal(result.iloc[:10], df)
+        tm.assert_frame_equal(result.iloc[10:], df)
+
+        # append
+        result = df.iloc[0:8, :]._append(df.iloc[8:])
+        tm.assert_frame_equal(result, df)
+
+        result = df.iloc[0:8, :]._append(df.iloc[8:9])._append(df.iloc[9:10])
+        tm.assert_frame_equal(result, df)
+
+        expected = concat([df, df], axis=0)
+        result = df._append(df)
+        tm.assert_frame_equal(result, expected)
+
+
+class TestMultiIndexConcat:
+    def test_concat_multiindex_with_keys(self, multiindex_dataframe_random_data):
+        frame = multiindex_dataframe_random_data
+        index = frame.index
+        result = concat([frame, frame], keys=[0, 1], names=["iteration"])
+
+        assert result.index.names == ("iteration",) + index.names
+        tm.assert_frame_equal(result.loc[0], frame)
+        tm.assert_frame_equal(result.loc[1], frame)
+        assert result.index.nlevels == 3
+
+    def test_concat_multiindex_with_none_in_index_names(self):
+        # GH 15787
+        index = MultiIndex.from_product([[1], range(5)], names=["level1", None])
+        df = DataFrame({"col": range(5)}, index=index, dtype=np.int32)
+
+        result = concat([df, df], keys=[1, 2], names=["level2"])
+        index = MultiIndex.from_product(
+            [[1, 2], [1], range(5)], names=["level2", "level1", None]
+        )
+        expected = DataFrame({"col": list(range(5)) * 2}, index=index, dtype=np.int32)
+        tm.assert_frame_equal(result, expected)
+
+        result = concat([df, df[:2]], keys=[1, 2], names=["level2"])
+        level2 = [1] * 5 + [2] * 2
+        level1 = [1] * 7
+        no_name = list(range(5)) + list(range(2))
+        tuples = list(zip(level2, level1, no_name))
+        index = MultiIndex.from_tuples(tuples, names=["level2", "level1", None])
+        expected = DataFrame({"col": no_name}, index=index, dtype=np.int32)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_multiindex_rangeindex(self):
+        # GH13542
+        # when multi-index levels are RangeIndex objects
+        # there is a bug in concat with objects of len 1
+
+        df = DataFrame(np.random.default_rng(2).standard_normal((9, 2)))
+        df.index = MultiIndex(
+            levels=[pd.RangeIndex(3), pd.RangeIndex(3)],
+            codes=[np.repeat(np.arange(3), 3), np.tile(np.arange(3), 3)],
+        )
+
+        res = concat([df.iloc[[2, 3, 4], :], df.iloc[[5], :]])
+        exp = df.iloc[[2, 3, 4, 5], :]
+        tm.assert_frame_equal(res, exp)
+
+    def test_concat_multiindex_dfs_with_deepcopy(self):
+        # GH 9967
+        example_multiindex1 = MultiIndex.from_product([["a"], ["b"]])
+        example_dataframe1 = DataFrame([0], index=example_multiindex1)
+
+        example_multiindex2 = MultiIndex.from_product([["a"], ["c"]])
+        example_dataframe2 = DataFrame([1], index=example_multiindex2)
+
+        example_dict = {"s1": example_dataframe1, "s2": example_dataframe2}
+        expected_index = MultiIndex(
+            levels=[["s1", "s2"], ["a"], ["b", "c"]],
+            codes=[[0, 1], [0, 0], [0, 1]],
+            names=["testname", None, None],
+        )
+        expected = DataFrame([[0], [1]], index=expected_index)
+        result_copy = concat(deepcopy(example_dict), names=["testname"])
+        tm.assert_frame_equal(result_copy, expected)
+        result_no_copy = concat(example_dict, names=["testname"])
+        tm.assert_frame_equal(result_no_copy, expected)
+
+    @pytest.mark.parametrize(
+        "mi1_list",
+        [
+            [["a"], range(2)],
+            [["b"], np.arange(2.0, 4.0)],
+            [["c"], ["A", "B"]],
+            [["d"], pd.date_range(start="2017", end="2018", periods=2)],
+        ],
+    )
+    @pytest.mark.parametrize(
+        "mi2_list",
+        [
+            [["a"], range(2)],
+            [["b"], np.arange(2.0, 4.0)],
+            [["c"], ["A", "B"]],
+            [["d"], pd.date_range(start="2017", end="2018", periods=2)],
+        ],
+    )
+    def test_concat_with_various_multiindex_dtypes(
+        self, mi1_list: list, mi2_list: list
+    ):
+        # GitHub #23478
+        mi1 = MultiIndex.from_product(mi1_list)
+        mi2 = MultiIndex.from_product(mi2_list)
+
+        df1 = DataFrame(np.zeros((1, len(mi1))), columns=mi1)
+        df2 = DataFrame(np.zeros((1, len(mi2))), columns=mi2)
+
+        if mi1_list[0] == mi2_list[0]:
+            expected_mi = MultiIndex(
+                levels=[mi1_list[0], list(mi1_list[1])],
+                codes=[[0, 0, 0, 0], [0, 1, 0, 1]],
+            )
+        else:
+            expected_mi = MultiIndex(
+                levels=[
+                    mi1_list[0] + mi2_list[0],
+                    list(mi1_list[1]) + list(mi2_list[1]),
+                ],
+                codes=[[0, 0, 1, 1], [0, 1, 2, 3]],
+            )
+
+        expected_df = DataFrame(np.zeros((1, len(expected_mi))), columns=expected_mi)
+
+        with tm.assert_produces_warning(None):
+            result_df = concat((df1, df2), axis=1)
+
+        tm.assert_frame_equal(expected_df, result_df)
+
+    def test_concat_multiindex_(self):
+        # GitHub #44786
+        df = DataFrame({"col": ["a", "b", "c"]}, index=["1", "2", "2"])
+        df = concat([df], keys=["X"])
+
+        iterables = [["X"], ["1", "2", "2"]]
+        result_index = df.index
+        expected_index = MultiIndex.from_product(iterables)
+
+        tm.assert_index_equal(result_index, expected_index)
+
+        result_df = df
+        expected_df = DataFrame(
+            {"col": ["a", "b", "c"]}, index=MultiIndex.from_product(iterables)
+        )
+        tm.assert_frame_equal(result_df, expected_df)
+
+    def test_concat_with_key_not_unique(self):
+        # GitHub #46519
+        df1 = DataFrame({"name": [1]})
+        df2 = DataFrame({"name": [2]})
+        df3 = DataFrame({"name": [3]})
+        df_a = concat([df1, df2, df3], keys=["x", "y", "x"])
+        # the warning is caused by indexing unsorted multi-index
+        with tm.assert_produces_warning(
+            PerformanceWarning, match="indexing past lexsort depth"
+        ):
+            out_a = df_a.loc[("x", 0), :]
+
+        df_b = DataFrame(
+            {"name": [1, 2, 3]}, index=Index([("x", 0), ("y", 0), ("x", 0)])
+        )
+        with tm.assert_produces_warning(
+            PerformanceWarning, match="indexing past lexsort depth"
+        ):
+            out_b = df_b.loc[("x", 0)]
+
+        tm.assert_frame_equal(out_a, out_b)
+
+        df1 = DataFrame({"name": ["a", "a", "b"]})
+        df2 = DataFrame({"name": ["a", "b"]})
+        df3 = DataFrame({"name": ["c", "d"]})
+        df_a = concat([df1, df2, df3], keys=["x", "y", "x"])
+        with tm.assert_produces_warning(
+            PerformanceWarning, match="indexing past lexsort depth"
+        ):
+            out_a = df_a.loc[("x", 0), :]
+
+        df_b = DataFrame(
+            {
+                "a": ["x", "x", "x", "y", "y", "x", "x"],
+                "b": [0, 1, 2, 0, 1, 0, 1],
+                "name": list("aababcd"),
+            }
+        ).set_index(["a", "b"])
+        df_b.index.names = [None, None]
+        with tm.assert_produces_warning(
+            PerformanceWarning, match="indexing past lexsort depth"
+        ):
+            out_b = df_b.loc[("x", 0), :]
+
+        tm.assert_frame_equal(out_a, out_b)
+
+    def test_concat_with_duplicated_levels(self):
+        # keyword levels should be unique
+        df1 = DataFrame({"A": [1]}, index=["x"])
+        df2 = DataFrame({"A": [1]}, index=["y"])
+        msg = r"Level values not unique: \['x', 'y', 'y'\]"
+        with pytest.raises(ValueError, match=msg):
+            concat([df1, df2], keys=["x", "y"], levels=[["x", "y", "y"]])
+
+    @pytest.mark.parametrize("levels", [[["x", "y"]], [["x", "y", "y"]]])
+    def test_concat_with_levels_with_none_keys(self, levels):
+        df1 = DataFrame({"A": [1]}, index=["x"])
+        df2 = DataFrame({"A": [1]}, index=["y"])
+        msg = "levels supported only when keys is not None"
+        with pytest.raises(ValueError, match=msg):
+            concat([df1, df2], levels=levels)
+
+    def test_concat_range_index_result(self):
+        # GH#47501
+        df1 = DataFrame({"a": [1, 2]})
+        df2 = DataFrame({"b": [1, 2]})
+
+        result = concat([df1, df2], sort=True, axis=1)
+        expected = DataFrame({"a": [1, 2], "b": [1, 2]})
+        tm.assert_frame_equal(result, expected)
+        expected_index = pd.RangeIndex(0, 2)
+        tm.assert_index_equal(result.index, expected_index, exact=True)
+
+    def test_concat_index_keep_dtype(self):
+        # GH#47329
+        df1 = DataFrame([[0, 1, 1]], columns=Index([1, 2, 3], dtype="object"))
+        df2 = DataFrame([[0, 1]], columns=Index([1, 2], dtype="object"))
+        result = concat([df1, df2], ignore_index=True, join="outer", sort=True)
+        expected = DataFrame(
+            [[0, 1, 1.0], [0, 1, np.nan]], columns=Index([1, 2, 3], dtype="object")
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_index_keep_dtype_ea_numeric(self, any_numeric_ea_dtype):
+        # GH#47329
+        df1 = DataFrame(
+            [[0, 1, 1]], columns=Index([1, 2, 3], dtype=any_numeric_ea_dtype)
+        )
+        df2 = DataFrame([[0, 1]], columns=Index([1, 2], dtype=any_numeric_ea_dtype))
+        result = concat([df1, df2], ignore_index=True, join="outer", sort=True)
+        expected = DataFrame(
+            [[0, 1, 1.0], [0, 1, np.nan]],
+            columns=Index([1, 2, 3], dtype=any_numeric_ea_dtype),
+        )
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("dtype", ["Int8", "Int16", "Int32"])
+    def test_concat_index_find_common(self, dtype):
+        # GH#47329
+        df1 = DataFrame([[0, 1, 1]], columns=Index([1, 2, 3], dtype=dtype))
+        df2 = DataFrame([[0, 1]], columns=Index([1, 2], dtype="Int32"))
+        result = concat([df1, df2], ignore_index=True, join="outer", sort=True)
+        expected = DataFrame(
+            [[0, 1, 1.0], [0, 1, np.nan]], columns=Index([1, 2, 3], dtype="Int32")
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_axis_1_sort_false_rangeindex(self, using_infer_string):
+        # GH 46675
+        s1 = Series(["a", "b", "c"])
+        s2 = Series(["a", "b"])
+        s3 = Series(["a", "b", "c", "d"])
+        s4 = Series(
+            [], dtype=object if not using_infer_string else "string[pyarrow_numpy]"
+        )
+        result = concat(
+            [s1, s2, s3, s4], sort=False, join="outer", ignore_index=False, axis=1
+        )
+        expected = DataFrame(
+            [
+                ["a"] * 3 + [np.nan],
+                ["b"] * 3 + [np.nan],
+                ["c", np.nan] * 2,
+                [np.nan] * 2 + ["d"] + [np.nan],
+            ],
+            dtype=object if not using_infer_string else "string[pyarrow_numpy]",
+        )
+        tm.assert_frame_equal(
+            result, expected, check_index_type=True, check_column_type=True
+        )

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_invalid.py

@@ -0,0 +1,54 @@
+from io import StringIO
+
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    concat,
+    read_csv,
+)
+import pandas._testing as tm
+
+
+class TestInvalidConcat:
+    @pytest.mark.parametrize("obj", [1, {}, [1, 2], (1, 2)])
+    def test_concat_invalid(self, obj):
+        # trying to concat a ndframe with a non-ndframe
+        df1 = DataFrame(range(2))
+        msg = (
+            f"cannot concatenate object of type '{type(obj)}'; "
+            "only Series and DataFrame objs are valid"
+        )
+        with pytest.raises(TypeError, match=msg):
+            concat([df1, obj])
+
+    def test_concat_invalid_first_argument(self):
+        df1 = DataFrame(range(2))
+        msg = (
+            "first argument must be an iterable of pandas "
+            'objects, you passed an object of type "DataFrame"'
+        )
+        with pytest.raises(TypeError, match=msg):
+            concat(df1)
+
+    def test_concat_generator_obj(self):
+        # generator ok though
+        concat(DataFrame(np.random.default_rng(2).random((5, 5))) for _ in range(3))
+
+    def test_concat_textreader_obj(self):
+        # text reader ok
+        # GH6583
+        data = """index,A,B,C,D
+                  foo,2,3,4,5
+                  bar,7,8,9,10
+                  baz,12,13,14,15
+                  qux,12,13,14,15
+                  foo2,12,13,14,15
+                  bar2,12,13,14,15
+               """
+
+        with read_csv(StringIO(data), chunksize=1) as reader:
+            result = concat(reader, ignore_index=True)
+        expected = read_csv(StringIO(data))
+        tm.assert_frame_equal(result, expected)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_series.py

@@ -0,0 +1,175 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    DatetimeIndex,
+    Index,
+    MultiIndex,
+    Series,
+    concat,
+    date_range,
+)
+import pandas._testing as tm
+
+
+class TestSeriesConcat:
+    def test_concat_series(self):
+        ts = Series(
+            np.arange(20, dtype=np.float64),
+            index=date_range("2020-01-01", periods=20),
+            name="foo",
+        )
+        ts.name = "foo"
+
+        pieces = [ts[:5], ts[5:15], ts[15:]]
+
+        result = concat(pieces)
+        tm.assert_series_equal(result, ts)
+        assert result.name == ts.name
+
+        result = concat(pieces, keys=[0, 1, 2])
+        expected = ts.copy()
+
+        ts.index = DatetimeIndex(np.array(ts.index.values, dtype="M8[ns]"))
+
+        exp_codes = [np.repeat([0, 1, 2], [len(x) for x in pieces]), np.arange(len(ts))]
+        exp_index = MultiIndex(levels=[[0, 1, 2], ts.index], codes=exp_codes)
+        expected.index = exp_index
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_empty_and_non_empty_series_regression(self):
+        # GH 18187 regression test
+        s1 = Series([1])
+        s2 = Series([], dtype=object)
+
+        expected = s1
+        msg = "The behavior of array concatenation with empty entries is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = concat([s1, s2])
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_series_axis1(self):
+        ts = Series(
+            np.arange(10, dtype=np.float64), index=date_range("2020-01-01", periods=10)
+        )
+
+        pieces = [ts[:-2], ts[2:], ts[2:-2]]
+
+        result = concat(pieces, axis=1)
+        expected = DataFrame(pieces).T
+        tm.assert_frame_equal(result, expected)
+
+        result = concat(pieces, keys=["A", "B", "C"], axis=1)
+        expected = DataFrame(pieces, index=["A", "B", "C"]).T
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_series_axis1_preserves_series_names(self):
+        # preserve series names, #2489
+        s = Series(np.random.default_rng(2).standard_normal(5), name="A")
+        s2 = Series(np.random.default_rng(2).standard_normal(5), name="B")
+
+        result = concat([s, s2], axis=1)
+        expected = DataFrame({"A": s, "B": s2})
+        tm.assert_frame_equal(result, expected)
+
+        s2.name = None
+        result = concat([s, s2], axis=1)
+        tm.assert_index_equal(result.columns, Index(["A", 0], dtype="object"))
+
+    def test_concat_series_axis1_with_reindex(self, sort):
+        # must reindex, #2603
+        s = Series(
+            np.random.default_rng(2).standard_normal(3), index=["c", "a", "b"], name="A"
+        )
+        s2 = Series(
+            np.random.default_rng(2).standard_normal(4),
+            index=["d", "a", "b", "c"],
+            name="B",
+        )
+        result = concat([s, s2], axis=1, sort=sort)
+        expected = DataFrame({"A": s, "B": s2}, index=["c", "a", "b", "d"])
+        if sort:
+            expected = expected.sort_index()
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_series_axis1_names_applied(self):
+        # ensure names argument is not ignored on axis=1, #23490
+        s = Series([1, 2, 3])
+        s2 = Series([4, 5, 6])
+        result = concat([s, s2], axis=1, keys=["a", "b"], names=["A"])
+        expected = DataFrame(
+            [[1, 4], [2, 5], [3, 6]], columns=Index(["a", "b"], name="A")
+        )
+        tm.assert_frame_equal(result, expected)
+
+        result = concat([s, s2], axis=1, keys=[("a", 1), ("b", 2)], names=["A", "B"])
+        expected = DataFrame(
+            [[1, 4], [2, 5], [3, 6]],
+            columns=MultiIndex.from_tuples([("a", 1), ("b", 2)], names=["A", "B"]),
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_series_axis1_same_names_ignore_index(self):
+        dates = date_range("01-Jan-2013", "01-Jan-2014", freq="MS")[0:-1]
+        s1 = Series(
+            np.random.default_rng(2).standard_normal(len(dates)),
+            index=dates,
+            name="value",
+        )
+        s2 = Series(
+            np.random.default_rng(2).standard_normal(len(dates)),
+            index=dates,
+            name="value",
+        )
+
+        result = concat([s1, s2], axis=1, ignore_index=True)
+        expected = Index(range(2))
+
+        tm.assert_index_equal(result.columns, expected, exact=True)
+
+    @pytest.mark.parametrize(
+        "s1name,s2name", [(np.int64(190), (43, 0)), (190, (43, 0))]
+    )
+    def test_concat_series_name_npscalar_tuple(self, s1name, s2name):
+        # GH21015
+        s1 = Series({"a": 1, "b": 2}, name=s1name)
+        s2 = Series({"c": 5, "d": 6}, name=s2name)
+        result = concat([s1, s2])
+        expected = Series({"a": 1, "b": 2, "c": 5, "d": 6})
+        tm.assert_series_equal(result, expected)
+
+    def test_concat_series_partial_columns_names(self):
+        # GH10698
+        named_series = Series([1, 2], name="foo")
+        unnamed_series1 = Series([1, 2])
+        unnamed_series2 = Series([4, 5])
+
+        result = concat([named_series, unnamed_series1, unnamed_series2], axis=1)
+        expected = DataFrame(
+            {"foo": [1, 2], 0: [1, 2], 1: [4, 5]}, columns=["foo", 0, 1]
+        )
+        tm.assert_frame_equal(result, expected)
+
+        result = concat(
+            [named_series, unnamed_series1, unnamed_series2],
+            axis=1,
+            keys=["red", "blue", "yellow"],
+        )
+        expected = DataFrame(
+            {"red": [1, 2], "blue": [1, 2], "yellow": [4, 5]},
+            columns=["red", "blue", "yellow"],
+        )
+        tm.assert_frame_equal(result, expected)
+
+        result = concat(
+            [named_series, unnamed_series1, unnamed_series2], axis=1, ignore_index=True
+        )
+        expected = DataFrame({0: [1, 2], 1: [1, 2], 2: [4, 5]})
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_series_length_one_reversed(self, frame_or_series):
+        # GH39401
+        obj = frame_or_series([100])
+        result = concat([obj.iloc[::-1]])
+        tm.assert_equal(result, obj)

venv/lib/python3.12/site-packages/pandas/tests/reshape/concat/test_sort.py

@@ -0,0 +1,118 @@
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import DataFrame
+import pandas._testing as tm
+
+
+class TestConcatSort:
+    def test_concat_sorts_columns(self, sort):
+        # GH-4588
+        df1 = DataFrame({"a": [1, 2], "b": [1, 2]}, columns=["b", "a"])
+        df2 = DataFrame({"a": [3, 4], "c": [5, 6]})
+
+        # for sort=True/None
+        expected = DataFrame(
+            {"a": [1, 2, 3, 4], "b": [1, 2, None, None], "c": [None, None, 5, 6]},
+            columns=["a", "b", "c"],
+        )
+
+        if sort is False:
+            expected = expected[["b", "a", "c"]]
+
+        # default
+        with tm.assert_produces_warning(None):
+            result = pd.concat([df1, df2], ignore_index=True, sort=sort)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_sorts_index(self, sort):
+        df1 = DataFrame({"a": [1, 2, 3]}, index=["c", "a", "b"])
+        df2 = DataFrame({"b": [1, 2]}, index=["a", "b"])
+
+        # For True/None
+        expected = DataFrame(
+            {"a": [2, 3, 1], "b": [1, 2, None]},
+            index=["a", "b", "c"],
+            columns=["a", "b"],
+        )
+        if sort is False:
+            expected = expected.loc[["c", "a", "b"]]
+
+        # Warn and sort by default
+        with tm.assert_produces_warning(None):
+            result = pd.concat([df1, df2], axis=1, sort=sort)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_inner_sort(self, sort):
+        # https://github.com/pandas-dev/pandas/pull/20613
+        df1 = DataFrame(
+            {"a": [1, 2], "b": [1, 2], "c": [1, 2]}, columns=["b", "a", "c"]
+        )
+        df2 = DataFrame({"a": [1, 2], "b": [3, 4]}, index=[3, 4])
+
+        with tm.assert_produces_warning(None):
+            # unset sort should *not* warn for inner join
+            # since that never sorted
+            result = pd.concat([df1, df2], sort=sort, join="inner", ignore_index=True)
+
+        expected = DataFrame({"b": [1, 2, 3, 4], "a": [1, 2, 1, 2]}, columns=["b", "a"])
+        if sort is True:
+            expected = expected[["a", "b"]]
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_aligned_sort(self):
+        # GH-4588
+        df = DataFrame({"c": [1, 2], "b": [3, 4], "a": [5, 6]}, columns=["c", "b", "a"])
+        result = pd.concat([df, df], sort=True, ignore_index=True)
+        expected = DataFrame(
+            {"a": [5, 6, 5, 6], "b": [3, 4, 3, 4], "c": [1, 2, 1, 2]},
+            columns=["a", "b", "c"],
+        )
+        tm.assert_frame_equal(result, expected)
+
+        result = pd.concat(
+            [df, df[["c", "b"]]], join="inner", sort=True, ignore_index=True
+        )
+        expected = expected[["b", "c"]]
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_aligned_sort_does_not_raise(self):
+        # GH-4588
+        # We catch TypeErrors from sorting internally and do not re-raise.
+        df = DataFrame({1: [1, 2], "a": [3, 4]}, columns=[1, "a"])
+        expected = DataFrame({1: [1, 2, 1, 2], "a": [3, 4, 3, 4]}, columns=[1, "a"])
+        result = pd.concat([df, df], ignore_index=True, sort=True)
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_frame_with_sort_false(self):
+        # GH 43375
+        result = pd.concat(
+            [DataFrame({i: i}, index=[i]) for i in range(2, 0, -1)], sort=False
+        )
+        expected = DataFrame([[2, np.nan], [np.nan, 1]], index=[2, 1], columns=[2, 1])
+
+        tm.assert_frame_equal(result, expected)
+
+        # GH 37937
+        df1 = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]}, index=[1, 2, 3])
+        df2 = DataFrame({"c": [7, 8, 9], "d": [10, 11, 12]}, index=[3, 1, 6])
+        result = pd.concat([df2, df1], axis=1, sort=False)
+        expected = DataFrame(
+            [
+                [7.0, 10.0, 3.0, 6.0],
+                [8.0, 11.0, 1.0, 4.0],
+                [9.0, 12.0, np.nan, np.nan],
+                [np.nan, np.nan, 2.0, 5.0],
+            ],
+            index=[3, 1, 6, 2],
+            columns=["c", "d", "a", "b"],
+        )
+        tm.assert_frame_equal(result, expected)
+
+    def test_concat_sort_none_raises(self):
+        # GH#41518
+        df = DataFrame({1: [1, 2], "a": [3, 4]})
+        msg = "The 'sort' keyword only accepts boolean values; None was passed."
+        with pytest.raises(ValueError, match=msg):
+            pd.concat([df, df], sort=None)