venv

venv/lib/python3.12/site-packages/pandas/tests/copy_view/index/test_datetimeindex.py

@@ -0,0 +1,69 @@
+import pytest
+
+from pandas import (
+    DatetimeIndex,
+    Series,
+    Timestamp,
+    date_range,
+)
+import pandas._testing as tm
+
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Setting a value on a view:FutureWarning"
+)
+
+
+@pytest.mark.parametrize(
+    "cons",
+    [
+        lambda x: DatetimeIndex(x),
+        lambda x: DatetimeIndex(DatetimeIndex(x)),
+    ],
+)
+def test_datetimeindex(using_copy_on_write, cons):
+    dt = date_range("2019-12-31", periods=3, freq="D")
+    ser = Series(dt)
+    idx = cons(ser)
+    expected = idx.copy(deep=True)
+    ser.iloc[0] = Timestamp("2020-12-31")
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)
+
+
+def test_datetimeindex_tz_convert(using_copy_on_write):
+    dt = date_range("2019-12-31", periods=3, freq="D", tz="Europe/Berlin")
+    ser = Series(dt)
+    idx = DatetimeIndex(ser).tz_convert("US/Eastern")
+    expected = idx.copy(deep=True)
+    ser.iloc[0] = Timestamp("2020-12-31", tz="Europe/Berlin")
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)
+
+
+def test_datetimeindex_tz_localize(using_copy_on_write):
+    dt = date_range("2019-12-31", periods=3, freq="D")
+    ser = Series(dt)
+    idx = DatetimeIndex(ser).tz_localize("Europe/Berlin")
+    expected = idx.copy(deep=True)
+    ser.iloc[0] = Timestamp("2020-12-31")
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)
+
+
+def test_datetimeindex_isocalendar(using_copy_on_write):
+    dt = date_range("2019-12-31", periods=3, freq="D")
+    ser = Series(dt)
+    df = DatetimeIndex(ser).isocalendar()
+    expected = df.index.copy(deep=True)
+    ser.iloc[0] = Timestamp("2020-12-31")
+    if using_copy_on_write:
+        tm.assert_index_equal(df.index, expected)
+
+
+def test_index_values(using_copy_on_write):
+    idx = date_range("2019-12-31", periods=3, freq="D")
+    result = idx.values
+    if using_copy_on_write:
+        assert result.flags.writeable is False
+    else:
+        assert result.flags.writeable is True

venv/lib/python3.12/site-packages/pandas/tests/copy_view/index/test_index.py

@@ -0,0 +1,184 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    Index,
+    Series,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+def index_view(index_data=[1, 2]):
+    df = DataFrame({"a": index_data, "b": 1.5})
+    view = df[:]
+    df = df.set_index("a", drop=True)
+    idx = df.index
+    # df = None
+    return idx, view
+
+
+def test_set_index_update_column(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2], "b": 1})
+    df = df.set_index("a", drop=False)
+    expected = df.index.copy(deep=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.iloc[0, 0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(df.index, expected)
+    else:
+        tm.assert_index_equal(df.index, Index([100, 2], name="a"))
+
+
+def test_set_index_drop_update_column(using_copy_on_write):
+    df = DataFrame({"a": [1, 2], "b": 1.5})
+    view = df[:]
+    df = df.set_index("a", drop=True)
+    expected = df.index.copy(deep=True)
+    view.iloc[0, 0] = 100
+    tm.assert_index_equal(df.index, expected)
+
+
+def test_set_index_series(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2], "b": 1.5})
+    ser = Series([10, 11])
+    df = df.set_index(ser)
+    expected = df.index.copy(deep=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(df.index, expected)
+    else:
+        tm.assert_index_equal(df.index, Index([100, 11]))
+
+
+def test_assign_index_as_series(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2], "b": 1.5})
+    ser = Series([10, 11])
+    df.index = ser
+    expected = df.index.copy(deep=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(df.index, expected)
+    else:
+        tm.assert_index_equal(df.index, Index([100, 11]))
+
+
+def test_assign_index_as_index(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2], "b": 1.5})
+    ser = Series([10, 11])
+    rhs_index = Index(ser)
+    df.index = rhs_index
+    rhs_index = None  # overwrite to clear reference
+    expected = df.index.copy(deep=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(df.index, expected)
+    else:
+        tm.assert_index_equal(df.index, Index([100, 11]))
+
+
+def test_index_from_series(using_copy_on_write, warn_copy_on_write):
+    ser = Series([1, 2])
+    idx = Index(ser)
+    expected = idx.copy(deep=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)
+    else:
+        tm.assert_index_equal(idx, Index([100, 2]))
+
+
+def test_index_from_series_copy(using_copy_on_write):
+    ser = Series([1, 2])
+    idx = Index(ser, copy=True)  # noqa: F841
+    arr = get_array(ser)
+    ser.iloc[0] = 100
+    assert np.shares_memory(get_array(ser), arr)
+
+
+def test_index_from_index(using_copy_on_write, warn_copy_on_write):
+    ser = Series([1, 2])
+    idx = Index(ser)
+    idx = Index(idx)
+    expected = idx.copy(deep=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)
+    else:
+        tm.assert_index_equal(idx, Index([100, 2]))
+
+
+@pytest.mark.parametrize(
+    "func",
+    [
+        lambda x: x._shallow_copy(x._values),
+        lambda x: x.view(),
+        lambda x: x.take([0, 1]),
+        lambda x: x.repeat([1, 1]),
+        lambda x: x[slice(0, 2)],
+        lambda x: x[[0, 1]],
+        lambda x: x._getitem_slice(slice(0, 2)),
+        lambda x: x.delete([]),
+        lambda x: x.rename("b"),
+        lambda x: x.astype("Int64", copy=False),
+    ],
+    ids=[
+        "_shallow_copy",
+        "view",
+        "take",
+        "repeat",
+        "getitem_slice",
+        "getitem_list",
+        "_getitem_slice",
+        "delete",
+        "rename",
+        "astype",
+    ],
+)
+def test_index_ops(using_copy_on_write, func, request):
+    idx, view_ = index_view()
+    expected = idx.copy(deep=True)
+    if "astype" in request.node.callspec.id:
+        expected = expected.astype("Int64")
+    idx = func(idx)
+    view_.iloc[0, 0] = 100
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected, check_names=False)
+
+
+def test_infer_objects(using_copy_on_write):
+    idx, view_ = index_view(["a", "b"])
+    expected = idx.copy(deep=True)
+    idx = idx.infer_objects(copy=False)
+    view_.iloc[0, 0] = "aaaa"
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected, check_names=False)
+
+
+def test_index_to_frame(using_copy_on_write):
+    idx = Index([1, 2, 3], name="a")
+    expected = idx.copy(deep=True)
+    df = idx.to_frame()
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "a"), idx._values)
+        assert not df._mgr._has_no_reference(0)
+    else:
+        assert not np.shares_memory(get_array(df, "a"), idx._values)
+
+    df.iloc[0, 0] = 100
+    tm.assert_index_equal(idx, expected)
+
+
+def test_index_values(using_copy_on_write):
+    idx = Index([1, 2, 3])
+    result = idx.values
+    if using_copy_on_write:
+        assert result.flags.writeable is False
+    else:
+        assert result.flags.writeable is True

venv/lib/python3.12/site-packages/pandas/tests/copy_view/index/test_periodindex.py

@@ -0,0 +1,30 @@
+import pytest
+
+from pandas import (
+    Period,
+    PeriodIndex,
+    Series,
+    period_range,
+)
+import pandas._testing as tm
+
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Setting a value on a view:FutureWarning"
+)
+
+
+@pytest.mark.parametrize(
+    "cons",
+    [
+        lambda x: PeriodIndex(x),
+        lambda x: PeriodIndex(PeriodIndex(x)),
+    ],
+)
+def test_periodindex(using_copy_on_write, cons):
+    dt = period_range("2019-12-31", periods=3, freq="D")
+    ser = Series(dt)
+    idx = cons(ser)
+    expected = idx.copy(deep=True)
+    ser.iloc[0] = Period("2020-12-31")
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/index/test_timedeltaindex.py

@@ -0,0 +1,30 @@
+import pytest
+
+from pandas import (
+    Series,
+    Timedelta,
+    TimedeltaIndex,
+    timedelta_range,
+)
+import pandas._testing as tm
+
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Setting a value on a view:FutureWarning"
+)
+
+
+@pytest.mark.parametrize(
+    "cons",
+    [
+        lambda x: TimedeltaIndex(x),
+        lambda x: TimedeltaIndex(TimedeltaIndex(x)),
+    ],
+)
+def test_timedeltaindex(using_copy_on_write, cons):
+    dt = timedelta_range("1 day", periods=3)
+    ser = Series(dt)
+    idx = cons(ser)
+    expected = idx.copy(deep=True)
+    ser.iloc[0] = Timedelta("5 days")
+    if using_copy_on_write:
+        tm.assert_index_equal(idx, expected)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_array.py

@@ -0,0 +1,190 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    Series,
+    date_range,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+# -----------------------------------------------------------------------------
+# Copy/view behaviour for accessing underlying array of Series/DataFrame
+
+
+@pytest.mark.parametrize(
+    "method",
+    [lambda ser: ser.values, lambda ser: np.asarray(ser)],
+    ids=["values", "asarray"],
+)
+def test_series_values(using_copy_on_write, method):
+    ser = Series([1, 2, 3], name="name")
+    ser_orig = ser.copy()
+
+    arr = method(ser)
+
+    if using_copy_on_write:
+        # .values still gives a view but is read-only
+        assert np.shares_memory(arr, get_array(ser, "name"))
+        assert arr.flags.writeable is False
+
+        # mutating series through arr therefore doesn't work
+        with pytest.raises(ValueError, match="read-only"):
+            arr[0] = 0
+        tm.assert_series_equal(ser, ser_orig)
+
+        # mutating the series itself still works
+        ser.iloc[0] = 0
+        assert ser.values[0] == 0
+    else:
+        assert arr.flags.writeable is True
+        arr[0] = 0
+        assert ser.iloc[0] == 0
+
+
+@pytest.mark.parametrize(
+    "method",
+    [lambda df: df.values, lambda df: np.asarray(df)],
+    ids=["values", "asarray"],
+)
+def test_dataframe_values(using_copy_on_write, using_array_manager, method):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    df_orig = df.copy()
+
+    arr = method(df)
+
+    if using_copy_on_write:
+        # .values still gives a view but is read-only
+        assert np.shares_memory(arr, get_array(df, "a"))
+        assert arr.flags.writeable is False
+
+        # mutating series through arr therefore doesn't work
+        with pytest.raises(ValueError, match="read-only"):
+            arr[0, 0] = 0
+        tm.assert_frame_equal(df, df_orig)
+
+        # mutating the series itself still works
+        df.iloc[0, 0] = 0
+        assert df.values[0, 0] == 0
+    else:
+        assert arr.flags.writeable is True
+        arr[0, 0] = 0
+        if not using_array_manager:
+            assert df.iloc[0, 0] == 0
+        else:
+            tm.assert_frame_equal(df, df_orig)
+
+
+def test_series_to_numpy(using_copy_on_write):
+    ser = Series([1, 2, 3], name="name")
+    ser_orig = ser.copy()
+
+    # default: copy=False, no dtype or NAs
+    arr = ser.to_numpy()
+    if using_copy_on_write:
+        # to_numpy still gives a view but is read-only
+        assert np.shares_memory(arr, get_array(ser, "name"))
+        assert arr.flags.writeable is False
+
+        # mutating series through arr therefore doesn't work
+        with pytest.raises(ValueError, match="read-only"):
+            arr[0] = 0
+        tm.assert_series_equal(ser, ser_orig)
+
+        # mutating the series itself still works
+        ser.iloc[0] = 0
+        assert ser.values[0] == 0
+    else:
+        assert arr.flags.writeable is True
+        arr[0] = 0
+        assert ser.iloc[0] == 0
+
+    # specify copy=False gives a writeable array
+    ser = Series([1, 2, 3], name="name")
+    arr = ser.to_numpy(copy=True)
+    assert not np.shares_memory(arr, get_array(ser, "name"))
+    assert arr.flags.writeable is True
+
+    # specifying a dtype that already causes a copy also gives a writeable array
+    ser = Series([1, 2, 3], name="name")
+    arr = ser.to_numpy(dtype="float64")
+    assert not np.shares_memory(arr, get_array(ser, "name"))
+    assert arr.flags.writeable is True
+
+
+@pytest.mark.parametrize("order", ["F", "C"])
+def test_ravel_read_only(using_copy_on_write, order):
+    ser = Series([1, 2, 3])
+    with tm.assert_produces_warning(FutureWarning, match="is deprecated"):
+        arr = ser.ravel(order=order)
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    assert np.shares_memory(get_array(ser), arr)
+
+
+def test_series_array_ea_dtypes(using_copy_on_write):
+    ser = Series([1, 2, 3], dtype="Int64")
+    arr = np.asarray(ser, dtype="int64")
+    assert np.shares_memory(arr, get_array(ser))
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    else:
+        assert arr.flags.writeable is True
+
+    arr = np.asarray(ser)
+    assert np.shares_memory(arr, get_array(ser))
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    else:
+        assert arr.flags.writeable is True
+
+
+def test_dataframe_array_ea_dtypes(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]}, dtype="Int64")
+    arr = np.asarray(df, dtype="int64")
+    assert np.shares_memory(arr, get_array(df, "a"))
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    else:
+        assert arr.flags.writeable is True
+
+    arr = np.asarray(df)
+    assert np.shares_memory(arr, get_array(df, "a"))
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    else:
+        assert arr.flags.writeable is True
+
+
+def test_dataframe_array_string_dtype(using_copy_on_write, using_array_manager):
+    df = DataFrame({"a": ["a", "b"]}, dtype="string")
+    arr = np.asarray(df)
+    if not using_array_manager:
+        assert np.shares_memory(arr, get_array(df, "a"))
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    else:
+        assert arr.flags.writeable is True
+
+
+def test_dataframe_multiple_numpy_dtypes():
+    df = DataFrame({"a": [1, 2, 3], "b": 1.5})
+    arr = np.asarray(df)
+    assert not np.shares_memory(arr, get_array(df, "a"))
+    assert arr.flags.writeable is True
+
+
+def test_values_is_ea(using_copy_on_write):
+    df = DataFrame({"a": date_range("2012-01-01", periods=3)})
+    arr = np.asarray(df)
+    if using_copy_on_write:
+        assert arr.flags.writeable is False
+    else:
+        assert arr.flags.writeable is True
+
+
+def test_empty_dataframe():
+    df = DataFrame()
+    arr = np.asarray(df)
+    assert arr.flags.writeable is True

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_astype.py

@@ -0,0 +1,260 @@
+import pickle
+
+import numpy as np
+import pytest
+
+from pandas.compat.pyarrow import pa_version_under12p0
+import pandas.util._test_decorators as td
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    Series,
+    Timestamp,
+    date_range,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+def test_astype_single_dtype(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": 1.5})
+    df_orig = df.copy()
+    df2 = df.astype("float64")
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    # mutating df2 triggers a copy-on-write for that column/block
+    df2.iloc[0, 2] = 5.5
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+    tm.assert_frame_equal(df, df_orig)
+
+    # mutating parent also doesn't update result
+    df2 = df.astype("float64")
+    df.iloc[0, 2] = 5.5
+    tm.assert_frame_equal(df2, df_orig.astype("float64"))
+
+
+@pytest.mark.parametrize("dtype", ["int64", "Int64"])
+@pytest.mark.parametrize("new_dtype", ["int64", "Int64", "int64[pyarrow]"])
+def test_astype_avoids_copy(using_copy_on_write, dtype, new_dtype):
+    if new_dtype == "int64[pyarrow]":
+        pytest.importorskip("pyarrow")
+    df = DataFrame({"a": [1, 2, 3]}, dtype=dtype)
+    df_orig = df.copy()
+    df2 = df.astype(new_dtype)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    # mutating df2 triggers a copy-on-write for that column/block
+    df2.iloc[0, 0] = 10
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+    # mutating parent also doesn't update result
+    df2 = df.astype(new_dtype)
+    df.iloc[0, 0] = 100
+    tm.assert_frame_equal(df2, df_orig.astype(new_dtype))
+
+
+@pytest.mark.parametrize("dtype", ["float64", "int32", "Int32", "int32[pyarrow]"])
+def test_astype_different_target_dtype(using_copy_on_write, dtype):
+    if dtype == "int32[pyarrow]":
+        pytest.importorskip("pyarrow")
+    df = DataFrame({"a": [1, 2, 3]})
+    df_orig = df.copy()
+    df2 = df.astype(dtype)
+
+    assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    if using_copy_on_write:
+        assert df2._mgr._has_no_reference(0)
+
+    df2.iloc[0, 0] = 5
+    tm.assert_frame_equal(df, df_orig)
+
+    # mutating parent also doesn't update result
+    df2 = df.astype(dtype)
+    df.iloc[0, 0] = 100
+    tm.assert_frame_equal(df2, df_orig.astype(dtype))
+
+
+@td.skip_array_manager_invalid_test
+def test_astype_numpy_to_ea():
+    ser = Series([1, 2, 3])
+    with pd.option_context("mode.copy_on_write", True):
+        result = ser.astype("Int64")
+    assert np.shares_memory(get_array(ser), get_array(result))
+
+
+@pytest.mark.parametrize(
+    "dtype, new_dtype", [("object", "string"), ("string", "object")]
+)
+def test_astype_string_and_object(using_copy_on_write, dtype, new_dtype):
+    df = DataFrame({"a": ["a", "b", "c"]}, dtype=dtype)
+    df_orig = df.copy()
+    df2 = df.astype(new_dtype)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    df2.iloc[0, 0] = "x"
+    tm.assert_frame_equal(df, df_orig)
+
+
+@pytest.mark.parametrize(
+    "dtype, new_dtype", [("object", "string"), ("string", "object")]
+)
+def test_astype_string_and_object_update_original(
+    using_copy_on_write, dtype, new_dtype
+):
+    df = DataFrame({"a": ["a", "b", "c"]}, dtype=dtype)
+    df2 = df.astype(new_dtype)
+    df_orig = df2.copy()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    df.iloc[0, 0] = "x"
+    tm.assert_frame_equal(df2, df_orig)
+
+
+def test_astype_string_copy_on_pickle_roundrip():
+    # https://github.com/pandas-dev/pandas/issues/54654
+    # ensure_string_array may alter array inplace
+    base = Series(np.array([(1, 2), None, 1], dtype="object"))
+    base_copy = pickle.loads(pickle.dumps(base))
+    base_copy.astype(str)
+    tm.assert_series_equal(base, base_copy)
+
+
+def test_astype_dict_dtypes(using_copy_on_write):
+    df = DataFrame(
+        {"a": [1, 2, 3], "b": [4, 5, 6], "c": Series([1.5, 1.5, 1.5], dtype="float64")}
+    )
+    df_orig = df.copy()
+    df2 = df.astype({"a": "float64", "c": "float64"})
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+        assert np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+        assert not np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    # mutating df2 triggers a copy-on-write for that column/block
+    df2.iloc[0, 2] = 5.5
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+
+    df2.iloc[0, 1] = 10
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_astype_different_datetime_resos(using_copy_on_write):
+    df = DataFrame({"a": date_range("2019-12-31", periods=2, freq="D")})
+    result = df.astype("datetime64[ms]")
+
+    assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+    if using_copy_on_write:
+        assert result._mgr._has_no_reference(0)
+
+
+def test_astype_different_timezones(using_copy_on_write):
+    df = DataFrame(
+        {"a": date_range("2019-12-31", periods=5, freq="D", tz="US/Pacific")}
+    )
+    result = df.astype("datetime64[ns, Europe/Berlin]")
+    if using_copy_on_write:
+        assert not result._mgr._has_no_reference(0)
+        assert np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+
+
+def test_astype_different_timezones_different_reso(using_copy_on_write):
+    df = DataFrame(
+        {"a": date_range("2019-12-31", periods=5, freq="D", tz="US/Pacific")}
+    )
+    result = df.astype("datetime64[ms, Europe/Berlin]")
+    if using_copy_on_write:
+        assert result._mgr._has_no_reference(0)
+        assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+
+
+def test_astype_arrow_timestamp(using_copy_on_write):
+    pytest.importorskip("pyarrow")
+    df = DataFrame(
+        {
+            "a": [
+                Timestamp("2020-01-01 01:01:01.000001"),
+                Timestamp("2020-01-01 01:01:01.000001"),
+            ]
+        },
+        dtype="M8[ns]",
+    )
+    result = df.astype("timestamp[ns][pyarrow]")
+    if using_copy_on_write:
+        assert not result._mgr._has_no_reference(0)
+        if pa_version_under12p0:
+            assert not np.shares_memory(
+                get_array(df, "a"), get_array(result, "a")._pa_array
+            )
+        else:
+            assert np.shares_memory(
+                get_array(df, "a"), get_array(result, "a")._pa_array
+            )
+
+
+def test_convert_dtypes_infer_objects(using_copy_on_write):
+    ser = Series(["a", "b", "c"])
+    ser_orig = ser.copy()
+    result = ser.convert_dtypes(
+        convert_integer=False,
+        convert_boolean=False,
+        convert_floating=False,
+        convert_string=False,
+    )
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(ser), get_array(result))
+    else:
+        assert not np.shares_memory(get_array(ser), get_array(result))
+
+    result.iloc[0] = "x"
+    tm.assert_series_equal(ser, ser_orig)
+
+
+def test_convert_dtypes(using_copy_on_write):
+    df = DataFrame({"a": ["a", "b"], "b": [1, 2], "c": [1.5, 2.5], "d": [True, False]})
+    df_orig = df.copy()
+    df2 = df.convert_dtypes()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+        assert np.shares_memory(get_array(df2, "d"), get_array(df, "d"))
+        assert np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+        assert np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+        assert not np.shares_memory(get_array(df2, "b"), get_array(df, "b"))
+        assert not np.shares_memory(get_array(df2, "c"), get_array(df, "c"))
+        assert not np.shares_memory(get_array(df2, "d"), get_array(df, "d"))
+
+    df2.iloc[0, 0] = "x"
+    tm.assert_frame_equal(df, df_orig)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_chained_assignment_deprecation.py

@@ -0,0 +1,174 @@
+import numpy as np
+import pytest
+
+from pandas.compat import PY311
+from pandas.errors import (
+    ChainedAssignmentError,
+    SettingWithCopyWarning,
+)
+
+from pandas import (
+    DataFrame,
+    option_context,
+)
+import pandas._testing as tm
+
+
+def test_methods_iloc_warn(using_copy_on_write):
+    if not using_copy_on_write:
+        df = DataFrame({"a": [1, 2, 3], "b": 1})
+        with tm.assert_cow_warning(match="A value"):
+            df.iloc[:, 0].replace(1, 5, inplace=True)
+
+        with tm.assert_cow_warning(match="A value"):
+            df.iloc[:, 0].fillna(1, inplace=True)
+
+        with tm.assert_cow_warning(match="A value"):
+            df.iloc[:, 0].interpolate(inplace=True)
+
+        with tm.assert_cow_warning(match="A value"):
+            df.iloc[:, 0].ffill(inplace=True)
+
+        with tm.assert_cow_warning(match="A value"):
+            df.iloc[:, 0].bfill(inplace=True)
+
+
+@pytest.mark.parametrize(
+    "func, args",
+    [
+        ("replace", (4, 5)),
+        ("fillna", (1,)),
+        ("interpolate", ()),
+        ("bfill", ()),
+        ("ffill", ()),
+    ],
+)
+def test_methods_iloc_getitem_item_cache(
+    func, args, using_copy_on_write, warn_copy_on_write
+):
+    # ensure we don't incorrectly raise chained assignment warning because
+    # of the item cache / iloc not setting the item cache
+    df_orig = DataFrame({"a": [1, 2, 3], "b": 1})
+
+    df = df_orig.copy()
+    ser = df.iloc[:, 0]
+    getattr(ser, func)(*args, inplace=True)
+
+    # parent that holds item_cache is dead, so don't increase ref count
+    df = df_orig.copy()
+    ser = df.copy()["a"]
+    getattr(ser, func)(*args, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    ser = df.iloc[:, 0]  # iloc creates a new object
+    getattr(ser, func)(*args, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    ser = df["a"]
+    getattr(ser, func)(*args, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    # TODO(CoW-warn) because of the usage of *args, this doesn't warn on Py3.11+
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error(not PY311):
+            getattr(df["a"], func)(*args, inplace=True)
+    else:
+        with tm.assert_cow_warning(not PY311, match="A value"):
+            getattr(df["a"], func)(*args, inplace=True)
+
+    df = df_orig.copy()
+    ser = df["a"]  # populate the item_cache and keep ref
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error(not PY311):
+            getattr(df["a"], func)(*args, inplace=True)
+    else:
+        # ideally also warns on the default mode, but the ser' _cacher
+        # messes up the refcount + even in warning mode this doesn't trigger
+        # the warning of Py3.1+ (see above)
+        with tm.assert_cow_warning(warn_copy_on_write and not PY311, match="A value"):
+            getattr(df["a"], func)(*args, inplace=True)
+
+
+def test_methods_iloc_getitem_item_cache_fillna(
+    using_copy_on_write, warn_copy_on_write
+):
+    # ensure we don't incorrectly raise chained assignment warning because
+    # of the item cache / iloc not setting the item cache
+    df_orig = DataFrame({"a": [1, 2, 3], "b": 1})
+
+    df = df_orig.copy()
+    ser = df.iloc[:, 0]
+    ser.fillna(1, inplace=True)
+
+    # parent that holds item_cache is dead, so don't increase ref count
+    df = df_orig.copy()
+    ser = df.copy()["a"]
+    ser.fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    ser = df.iloc[:, 0]  # iloc creates a new object
+    ser.fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    ser = df["a"]
+    ser.fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].fillna(1, inplace=True)
+    else:
+        with tm.assert_cow_warning(match="A value"):
+            df["a"].fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    ser = df["a"]  # populate the item_cache and keep ref
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].fillna(1, inplace=True)
+    else:
+        # TODO(CoW-warn) ideally also warns on the default mode, but the ser' _cacher
+        # messes up the refcount
+        with tm.assert_cow_warning(warn_copy_on_write, match="A value"):
+            df["a"].fillna(1, inplace=True)
+
+
+# TODO(CoW-warn) expand the cases
+@pytest.mark.parametrize(
+    "indexer", [0, [0, 1], slice(0, 2), np.array([True, False, True])]
+)
+def test_series_setitem(indexer, using_copy_on_write, warn_copy_on_write):
+    # ensure we only get a single warning for those typical cases of chained
+    # assignment
+    df = DataFrame({"a": [1, 2, 3], "b": 1})
+
+    # using custom check instead of tm.assert_produces_warning because that doesn't
+    # fail if multiple warnings are raised
+    with pytest.warns() as record:
+        df["a"][indexer] = 0
+    assert len(record) == 1
+    if using_copy_on_write:
+        assert record[0].category == ChainedAssignmentError
+    else:
+        assert record[0].category == FutureWarning
+        assert "ChainedAssignmentError" in record[0].message.args[0]
+
+
+@pytest.mark.filterwarnings("ignore::pandas.errors.SettingWithCopyWarning")
+@pytest.mark.parametrize(
+    "indexer", ["a", ["a", "b"], slice(0, 2), np.array([True, False, True])]
+)
+def test_frame_setitem(indexer, using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3, 4, 5], "b": 1})
+
+    extra_warnings = () if using_copy_on_write else (SettingWithCopyWarning,)
+
+    with option_context("chained_assignment", "warn"):
+        with tm.raises_chained_assignment_error(extra_warnings=extra_warnings):
+            df[0:3][indexer] = 10

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_clip.py

@@ -0,0 +1,101 @@
+import numpy as np
+
+from pandas import (
+    DataFrame,
+    option_context,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+def test_clip_inplace_reference(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    df_copy = df.copy()
+    arr_a = get_array(df, "a")
+    view = df[:]
+    if warn_copy_on_write:
+        with tm.assert_cow_warning():
+            df.clip(lower=2, inplace=True)
+    else:
+        df.clip(lower=2, inplace=True)
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr_a)
+        assert df._mgr._has_no_reference(0)
+        assert view._mgr._has_no_reference(0)
+        tm.assert_frame_equal(df_copy, view)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr_a)
+
+
+def test_clip_inplace_reference_no_op(using_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    df_copy = df.copy()
+    arr_a = get_array(df, "a")
+    view = df[:]
+    df.clip(lower=0, inplace=True)
+
+    assert np.shares_memory(get_array(df, "a"), arr_a)
+
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+        assert not view._mgr._has_no_reference(0)
+        tm.assert_frame_equal(df_copy, view)
+
+
+def test_clip_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    arr_a = get_array(df, "a")
+    df.clip(lower=2, inplace=True)
+
+    assert np.shares_memory(get_array(df, "a"), arr_a)
+
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+def test_clip(using_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    df_orig = df.copy()
+    df2 = df.clip(lower=2)
+
+    assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+    tm.assert_frame_equal(df_orig, df)
+
+
+def test_clip_no_op(using_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    df2 = df.clip(lower=0)
+
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+
+def test_clip_chained_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1, 4, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].clip(1, 2, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            df[["a"]].clip(1, 2, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].clip(1, 2, inplace=True)
+
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                df[["a"]].clip(1, 2, inplace=True)
+
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                df[df["a"] > 1].clip(1, 2, inplace=True)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_constructors.py

@@ -0,0 +1,382 @@
+import numpy as np
+import pytest
+
+import pandas as pd
+from pandas import (
+    DataFrame,
+    DatetimeIndex,
+    Index,
+    Period,
+    PeriodIndex,
+    Series,
+    Timedelta,
+    TimedeltaIndex,
+    Timestamp,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+# -----------------------------------------------------------------------------
+# Copy/view behaviour for Series / DataFrame constructors
+
+
+@pytest.mark.parametrize("dtype", [None, "int64"])
+def test_series_from_series(dtype, using_copy_on_write, warn_copy_on_write):
+    # Case: constructing a Series from another Series object follows CoW rules:
+    # a new object is returned and thus mutations are not propagated
+    ser = Series([1, 2, 3], name="name")
+
+    # default is copy=False -> new Series is a shallow copy / view of original
+    result = Series(ser, dtype=dtype)
+
+    # the shallow copy still shares memory
+    assert np.shares_memory(get_array(ser), get_array(result))
+
+    if using_copy_on_write:
+        assert result._mgr.blocks[0].refs.has_reference()
+
+    if using_copy_on_write:
+        # mutating new series copy doesn't mutate original
+        result.iloc[0] = 0
+        assert ser.iloc[0] == 1
+        # mutating triggered a copy-on-write -> no longer shares memory
+        assert not np.shares_memory(get_array(ser), get_array(result))
+    else:
+        # mutating shallow copy does mutate original
+        with tm.assert_cow_warning(warn_copy_on_write):
+            result.iloc[0] = 0
+        assert ser.iloc[0] == 0
+        # and still shares memory
+        assert np.shares_memory(get_array(ser), get_array(result))
+
+    # the same when modifying the parent
+    result = Series(ser, dtype=dtype)
+
+    if using_copy_on_write:
+        # mutating original doesn't mutate new series
+        ser.iloc[0] = 0
+        assert result.iloc[0] == 1
+    else:
+        # mutating original does mutate shallow copy
+        with tm.assert_cow_warning(warn_copy_on_write):
+            ser.iloc[0] = 0
+        assert result.iloc[0] == 0
+
+
+def test_series_from_series_with_reindex(using_copy_on_write, warn_copy_on_write):
+    # Case: constructing a Series from another Series with specifying an index
+    # that potentially requires a reindex of the values
+    ser = Series([1, 2, 3], name="name")
+
+    # passing an index that doesn't actually require a reindex of the values
+    # -> without CoW we get an actual mutating view
+    for index in [
+        ser.index,
+        ser.index.copy(),
+        list(ser.index),
+        ser.index.rename("idx"),
+    ]:
+        result = Series(ser, index=index)
+        assert np.shares_memory(ser.values, result.values)
+        with tm.assert_cow_warning(warn_copy_on_write):
+            result.iloc[0] = 0
+        if using_copy_on_write:
+            assert ser.iloc[0] == 1
+        else:
+            assert ser.iloc[0] == 0
+
+    # ensure that if an actual reindex is needed, we don't have any refs
+    # (mutating the result wouldn't trigger CoW)
+    result = Series(ser, index=[0, 1, 2, 3])
+    assert not np.shares_memory(ser.values, result.values)
+    if using_copy_on_write:
+        assert not result._mgr.blocks[0].refs.has_reference()
+
+
+@pytest.mark.parametrize("fastpath", [False, True])
+@pytest.mark.parametrize("dtype", [None, "int64"])
+@pytest.mark.parametrize("idx", [None, pd.RangeIndex(start=0, stop=3, step=1)])
+@pytest.mark.parametrize(
+    "arr", [np.array([1, 2, 3], dtype="int64"), pd.array([1, 2, 3], dtype="Int64")]
+)
+def test_series_from_array(using_copy_on_write, idx, dtype, fastpath, arr):
+    if idx is None or dtype is not None:
+        fastpath = False
+    msg = "The 'fastpath' keyword in pd.Series is deprecated"
+    with tm.assert_produces_warning(DeprecationWarning, match=msg):
+        ser = Series(arr, dtype=dtype, index=idx, fastpath=fastpath)
+    ser_orig = ser.copy()
+    data = getattr(arr, "_data", arr)
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(ser), data)
+    else:
+        assert np.shares_memory(get_array(ser), data)
+
+    arr[0] = 100
+    if using_copy_on_write:
+        tm.assert_series_equal(ser, ser_orig)
+    else:
+        expected = Series([100, 2, 3], dtype=dtype if dtype is not None else arr.dtype)
+        tm.assert_series_equal(ser, expected)
+
+
+@pytest.mark.parametrize("copy", [True, False, None])
+def test_series_from_array_different_dtype(using_copy_on_write, copy):
+    arr = np.array([1, 2, 3], dtype="int64")
+    ser = Series(arr, dtype="int32", copy=copy)
+    assert not np.shares_memory(get_array(ser), arr)
+
+
+@pytest.mark.parametrize(
+    "idx",
+    [
+        Index([1, 2]),
+        DatetimeIndex([Timestamp("2019-12-31"), Timestamp("2020-12-31")]),
+        PeriodIndex([Period("2019-12-31"), Period("2020-12-31")]),
+        TimedeltaIndex([Timedelta("1 days"), Timedelta("2 days")]),
+    ],
+)
+def test_series_from_index(using_copy_on_write, idx):
+    ser = Series(idx)
+    expected = idx.copy(deep=True)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(ser), get_array(idx))
+        assert not ser._mgr._has_no_reference(0)
+    else:
+        assert not np.shares_memory(get_array(ser), get_array(idx))
+    ser.iloc[0] = ser.iloc[1]
+    tm.assert_index_equal(idx, expected)
+
+
+def test_series_from_index_different_dtypes(using_copy_on_write):
+    idx = Index([1, 2, 3], dtype="int64")
+    ser = Series(idx, dtype="int32")
+    assert not np.shares_memory(get_array(ser), get_array(idx))
+    if using_copy_on_write:
+        assert ser._mgr._has_no_reference(0)
+
+
+@pytest.mark.filterwarnings("ignore:Setting a value on a view:FutureWarning")
+@pytest.mark.parametrize("fastpath", [False, True])
+@pytest.mark.parametrize("dtype", [None, "int64"])
+@pytest.mark.parametrize("idx", [None, pd.RangeIndex(start=0, stop=3, step=1)])
+def test_series_from_block_manager(using_copy_on_write, idx, dtype, fastpath):
+    ser = Series([1, 2, 3], dtype="int64")
+    ser_orig = ser.copy()
+    msg = "The 'fastpath' keyword in pd.Series is deprecated"
+    with tm.assert_produces_warning(DeprecationWarning, match=msg):
+        ser2 = Series(ser._mgr, dtype=dtype, fastpath=fastpath, index=idx)
+    assert np.shares_memory(get_array(ser), get_array(ser2))
+    if using_copy_on_write:
+        assert not ser2._mgr._has_no_reference(0)
+
+    ser2.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_series_equal(ser, ser_orig)
+    else:
+        expected = Series([100, 2, 3])
+        tm.assert_series_equal(ser, expected)
+
+
+def test_series_from_block_manager_different_dtype(using_copy_on_write):
+    ser = Series([1, 2, 3], dtype="int64")
+    msg = "Passing a SingleBlockManager to Series"
+    with tm.assert_produces_warning(DeprecationWarning, match=msg):
+        ser2 = Series(ser._mgr, dtype="int32")
+    assert not np.shares_memory(get_array(ser), get_array(ser2))
+    if using_copy_on_write:
+        assert ser2._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize("use_mgr", [True, False])
+@pytest.mark.parametrize("columns", [None, ["a"]])
+def test_dataframe_constructor_mgr_or_df(
+    using_copy_on_write, warn_copy_on_write, columns, use_mgr
+):
+    df = DataFrame({"a": [1, 2, 3]})
+    df_orig = df.copy()
+
+    if use_mgr:
+        data = df._mgr
+        warn = DeprecationWarning
+    else:
+        data = df
+        warn = None
+    msg = "Passing a BlockManager to DataFrame"
+    with tm.assert_produces_warning(warn, match=msg, check_stacklevel=False):
+        new_df = DataFrame(data)
+
+    assert np.shares_memory(get_array(df, "a"), get_array(new_df, "a"))
+    with tm.assert_cow_warning(warn_copy_on_write and not use_mgr):
+        new_df.iloc[0] = 100
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), get_array(new_df, "a"))
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), get_array(new_df, "a"))
+        tm.assert_frame_equal(df, new_df)
+
+
+@pytest.mark.parametrize("dtype", [None, "int64", "Int64"])
+@pytest.mark.parametrize("index", [None, [0, 1, 2]])
+@pytest.mark.parametrize("columns", [None, ["a", "b"], ["a", "b", "c"]])
+def test_dataframe_from_dict_of_series(
+    request, using_copy_on_write, warn_copy_on_write, columns, index, dtype
+):
+    # Case: constructing a DataFrame from Series objects with copy=False
+    # has to do a lazy following CoW rules
+    # (the default for DataFrame(dict) is still to copy to ensure consolidation)
+    s1 = Series([1, 2, 3])
+    s2 = Series([4, 5, 6])
+    s1_orig = s1.copy()
+    expected = DataFrame(
+        {"a": [1, 2, 3], "b": [4, 5, 6]}, index=index, columns=columns, dtype=dtype
+    )
+
+    result = DataFrame(
+        {"a": s1, "b": s2}, index=index, columns=columns, dtype=dtype, copy=False
+    )
+
+    # the shallow copy still shares memory
+    assert np.shares_memory(get_array(result, "a"), get_array(s1))
+
+    # mutating the new dataframe doesn't mutate original
+    with tm.assert_cow_warning(warn_copy_on_write):
+        result.iloc[0, 0] = 10
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(s1))
+        tm.assert_series_equal(s1, s1_orig)
+    else:
+        assert s1.iloc[0] == 10
+
+    # the same when modifying the parent series
+    s1 = Series([1, 2, 3])
+    s2 = Series([4, 5, 6])
+    result = DataFrame(
+        {"a": s1, "b": s2}, index=index, columns=columns, dtype=dtype, copy=False
+    )
+    with tm.assert_cow_warning(warn_copy_on_write):
+        s1.iloc[0] = 10
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(s1))
+        tm.assert_frame_equal(result, expected)
+    else:
+        assert result.iloc[0, 0] == 10
+
+
+@pytest.mark.parametrize("dtype", [None, "int64"])
+def test_dataframe_from_dict_of_series_with_reindex(dtype):
+    # Case: constructing a DataFrame from Series objects with copy=False
+    # and passing an index that requires an actual (no-view) reindex -> need
+    # to ensure the result doesn't have refs set up to unnecessarily trigger
+    # a copy on write
+    s1 = Series([1, 2, 3])
+    s2 = Series([4, 5, 6])
+    df = DataFrame({"a": s1, "b": s2}, index=[1, 2, 3], dtype=dtype, copy=False)
+
+    # df should own its memory, so mutating shouldn't trigger a copy
+    arr_before = get_array(df, "a")
+    assert not np.shares_memory(arr_before, get_array(s1))
+    df.iloc[0, 0] = 100
+    arr_after = get_array(df, "a")
+    assert np.shares_memory(arr_before, arr_after)
+
+
+@pytest.mark.parametrize("cons", [Series, Index])
+@pytest.mark.parametrize(
+    "data, dtype", [([1, 2], None), ([1, 2], "int64"), (["a", "b"], None)]
+)
+def test_dataframe_from_series_or_index(
+    using_copy_on_write, warn_copy_on_write, data, dtype, cons
+):
+    obj = cons(data, dtype=dtype)
+    obj_orig = obj.copy()
+    df = DataFrame(obj, dtype=dtype)
+    assert np.shares_memory(get_array(obj), get_array(df, 0))
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.iloc[0, 0] = data[-1]
+    if using_copy_on_write:
+        tm.assert_equal(obj, obj_orig)
+
+
+@pytest.mark.parametrize("cons", [Series, Index])
+def test_dataframe_from_series_or_index_different_dtype(using_copy_on_write, cons):
+    obj = cons([1, 2], dtype="int64")
+    df = DataFrame(obj, dtype="int32")
+    assert not np.shares_memory(get_array(obj), get_array(df, 0))
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+def test_dataframe_from_series_infer_datetime(using_copy_on_write):
+    ser = Series([Timestamp("2019-12-31"), Timestamp("2020-12-31")], dtype=object)
+    with tm.assert_produces_warning(FutureWarning, match="Dtype inference"):
+        df = DataFrame(ser)
+    assert not np.shares_memory(get_array(ser), get_array(df, 0))
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize("index", [None, [0, 1, 2]])
+def test_dataframe_from_dict_of_series_with_dtype(index):
+    # Variant of above, but now passing a dtype that causes a copy
+    # -> need to ensure the result doesn't have refs set up to unnecessarily
+    # trigger a copy on write
+    s1 = Series([1.0, 2.0, 3.0])
+    s2 = Series([4, 5, 6])
+    df = DataFrame({"a": s1, "b": s2}, index=index, dtype="int64", copy=False)
+
+    # df should own its memory, so mutating shouldn't trigger a copy
+    arr_before = get_array(df, "a")
+    assert not np.shares_memory(arr_before, get_array(s1))
+    df.iloc[0, 0] = 100
+    arr_after = get_array(df, "a")
+    assert np.shares_memory(arr_before, arr_after)
+
+
+@pytest.mark.parametrize("copy", [False, None, True])
+def test_frame_from_numpy_array(using_copy_on_write, copy, using_array_manager):
+    arr = np.array([[1, 2], [3, 4]])
+    df = DataFrame(arr, copy=copy)
+
+    if (
+        using_copy_on_write
+        and copy is not False
+        or copy is True
+        or (using_array_manager and copy is None)
+    ):
+        assert not np.shares_memory(get_array(df, 0), arr)
+    else:
+        assert np.shares_memory(get_array(df, 0), arr)
+
+
+def test_dataframe_from_records_with_dataframe(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]})
+    df_orig = df.copy()
+    with tm.assert_produces_warning(FutureWarning):
+        df2 = DataFrame.from_records(df)
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+    assert np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df2.iloc[0, 0] = 100
+    if using_copy_on_write:
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        tm.assert_frame_equal(df, df2)
+
+
+def test_frame_from_dict_of_index(using_copy_on_write):
+    idx = Index([1, 2, 3])
+    expected = idx.copy(deep=True)
+    df = DataFrame({"a": idx}, copy=False)
+    assert np.shares_memory(get_array(df, "a"), idx._values)
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+
+        df.iloc[0, 0] = 100
+        tm.assert_index_equal(idx, expected)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_core_functionalities.py

@@ -0,0 +1,106 @@
+import numpy as np
+import pytest
+
+from pandas import DataFrame
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+def test_assigning_to_same_variable_removes_references(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]})
+    df = df.reset_index()
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(1)
+    arr = get_array(df, "a")
+    df.iloc[0, 1] = 100  # Write into a
+
+    assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_setitem_dont_track_unnecessary_references(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": 1, "c": 1})
+
+    df["b"] = 100
+    arr = get_array(df, "a")
+    # We split the block in setitem, if we are not careful the new blocks will
+    # reference each other triggering a copy
+    df.iloc[0, 0] = 100
+    assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_setitem_with_view_copies(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": 1, "c": 1})
+    view = df[:]
+    expected = df.copy()
+
+    df["b"] = 100
+    arr = get_array(df, "a")
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.iloc[0, 0] = 100  # Check that we correctly track reference
+    if using_copy_on_write:
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        tm.assert_frame_equal(view, expected)
+
+
+def test_setitem_with_view_invalidated_does_not_copy(
+    using_copy_on_write, warn_copy_on_write, request
+):
+    df = DataFrame({"a": [1, 2, 3], "b": 1, "c": 1})
+    view = df[:]
+
+    df["b"] = 100
+    arr = get_array(df, "a")
+    view = None  # noqa: F841
+    # TODO(CoW-warn) false positive? -> block gets split because of `df["b"] = 100`
+    # which introduces additional refs, even when those of `view` go out of scopes
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.iloc[0, 0] = 100
+    if using_copy_on_write:
+        # Setitem split the block. Since the old block shared data with view
+        # all the new blocks are referencing view and each other. When view
+        # goes out of scope, they don't share data with any other block,
+        # so we should not trigger a copy
+        mark = pytest.mark.xfail(
+            reason="blk.delete does not track references correctly"
+        )
+        request.applymarker(mark)
+        assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_out_of_scope(using_copy_on_write):
+    def func():
+        df = DataFrame({"a": [1, 2], "b": 1.5, "c": 1})
+        # create some subset
+        result = df[["a", "b"]]
+        return result
+
+    result = func()
+    if using_copy_on_write:
+        assert not result._mgr.blocks[0].refs.has_reference()
+        assert not result._mgr.blocks[1].refs.has_reference()
+
+
+def test_delete(using_copy_on_write):
+    df = DataFrame(
+        np.random.default_rng(2).standard_normal((4, 3)), columns=["a", "b", "c"]
+    )
+    del df["b"]
+    if using_copy_on_write:
+        assert not df._mgr.blocks[0].refs.has_reference()
+        assert not df._mgr.blocks[1].refs.has_reference()
+
+    df = df[["a"]]
+    if using_copy_on_write:
+        assert not df._mgr.blocks[0].refs.has_reference()
+
+
+def test_delete_reference(using_copy_on_write):
+    df = DataFrame(
+        np.random.default_rng(2).standard_normal((4, 3)), columns=["a", "b", "c"]
+    )
+    x = df[:]
+    del df["b"]
+    if using_copy_on_write:
+        assert df._mgr.blocks[0].refs.has_reference()
+        assert df._mgr.blocks[1].refs.has_reference()
+        assert x._mgr.blocks[0].refs.has_reference()

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_functions.py

@@ -0,0 +1,396 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    Index,
+    Series,
+    concat,
+    merge,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+def test_concat_frames(using_copy_on_write):
+    df = DataFrame({"b": ["a"] * 3})
+    df2 = DataFrame({"a": ["a"] * 3})
+    df_orig = df.copy()
+    result = concat([df, df2], axis=1)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "b"), get_array(df, "b"))
+        assert np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df, "b"))
+        assert not np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+
+    result.iloc[0, 0] = "d"
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df, "b"))
+        assert np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+
+    result.iloc[0, 1] = "d"
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_concat_frames_updating_input(using_copy_on_write):
+    df = DataFrame({"b": ["a"] * 3})
+    df2 = DataFrame({"a": ["a"] * 3})
+    result = concat([df, df2], axis=1)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "b"), get_array(df, "b"))
+        assert np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df, "b"))
+        assert not np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+
+    expected = result.copy()
+    df.iloc[0, 0] = "d"
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df, "b"))
+        assert np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+
+    df2.iloc[0, 0] = "d"
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df2, "a"))
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_series(using_copy_on_write):
+    ser = Series([1, 2], name="a")
+    ser2 = Series([3, 4], name="b")
+    ser_orig = ser.copy()
+    ser2_orig = ser2.copy()
+    result = concat([ser, ser2], axis=1)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), ser.values)
+        assert np.shares_memory(get_array(result, "b"), ser2.values)
+    else:
+        assert not np.shares_memory(get_array(result, "a"), ser.values)
+        assert not np.shares_memory(get_array(result, "b"), ser2.values)
+
+    result.iloc[0, 0] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), ser.values)
+        assert np.shares_memory(get_array(result, "b"), ser2.values)
+
+    result.iloc[0, 1] = 1000
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), ser2.values)
+    tm.assert_series_equal(ser, ser_orig)
+    tm.assert_series_equal(ser2, ser2_orig)
+
+
+def test_concat_frames_chained(using_copy_on_write):
+    df1 = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+    df2 = DataFrame({"c": [4, 5, 6]})
+    df3 = DataFrame({"d": [4, 5, 6]})
+    result = concat([concat([df1, df2], axis=1), df3], axis=1)
+    expected = result.copy()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "c"), get_array(df2, "c"))
+        assert np.shares_memory(get_array(result, "d"), get_array(df3, "d"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "c"), get_array(df2, "c"))
+        assert not np.shares_memory(get_array(result, "d"), get_array(df3, "d"))
+
+    df1.iloc[0, 0] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_series_chained(using_copy_on_write):
+    ser1 = Series([1, 2, 3], name="a")
+    ser2 = Series([4, 5, 6], name="c")
+    ser3 = Series([4, 5, 6], name="d")
+    result = concat([concat([ser1, ser2], axis=1), ser3], axis=1)
+    expected = result.copy()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(ser1, "a"))
+        assert np.shares_memory(get_array(result, "c"), get_array(ser2, "c"))
+        assert np.shares_memory(get_array(result, "d"), get_array(ser3, "d"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(ser1, "a"))
+        assert not np.shares_memory(get_array(result, "c"), get_array(ser2, "c"))
+        assert not np.shares_memory(get_array(result, "d"), get_array(ser3, "d"))
+
+    ser1.iloc[0] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(ser1, "a"))
+
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_series_updating_input(using_copy_on_write):
+    ser = Series([1, 2], name="a")
+    ser2 = Series([3, 4], name="b")
+    expected = DataFrame({"a": [1, 2], "b": [3, 4]})
+    result = concat([ser, ser2], axis=1)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(ser, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(ser2, "b"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(ser, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(ser2, "b"))
+
+    ser.iloc[0] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(ser, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(ser2, "b"))
+    tm.assert_frame_equal(result, expected)
+
+    ser2.iloc[0] = 1000
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(ser2, "b"))
+    tm.assert_frame_equal(result, expected)
+
+
+def test_concat_mixed_series_frame(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "c": 1})
+    ser = Series([4, 5, 6], name="d")
+    result = concat([df, ser], axis=1)
+    expected = result.copy()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+        assert np.shares_memory(get_array(result, "c"), get_array(df, "c"))
+        assert np.shares_memory(get_array(result, "d"), get_array(ser, "d"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+        assert not np.shares_memory(get_array(result, "c"), get_array(df, "c"))
+        assert not np.shares_memory(get_array(result, "d"), get_array(ser, "d"))
+
+    ser.iloc[0] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "d"), get_array(ser, "d"))
+
+    df.iloc[0, 0] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize("copy", [True, None, False])
+def test_concat_copy_keyword(using_copy_on_write, copy):
+    df = DataFrame({"a": [1, 2]})
+    df2 = DataFrame({"b": [1.5, 2.5]})
+
+    result = concat([df, df2], axis=1, copy=copy)
+
+    if using_copy_on_write or copy is False:
+        assert np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+        assert np.shares_memory(get_array(df2, "b"), get_array(result, "b"))
+    else:
+        assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+        assert not np.shares_memory(get_array(df2, "b"), get_array(result, "b"))
+
+
+@pytest.mark.parametrize(
+    "func",
+    [
+        lambda df1, df2, **kwargs: df1.merge(df2, **kwargs),
+        lambda df1, df2, **kwargs: merge(df1, df2, **kwargs),
+    ],
+)
+def test_merge_on_key(using_copy_on_write, func):
+    df1 = DataFrame({"key": ["a", "b", "c"], "a": [1, 2, 3]})
+    df2 = DataFrame({"key": ["a", "b", "c"], "b": [4, 5, 6]})
+    df1_orig = df1.copy()
+    df2_orig = df2.copy()
+
+    result = func(df1, df2, on="key")
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+        assert np.shares_memory(get_array(result, "key"), get_array(df1, "key"))
+        assert not np.shares_memory(get_array(result, "key"), get_array(df2, "key"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    result.iloc[0, 1] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    result.iloc[0, 2] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+    tm.assert_frame_equal(df1, df1_orig)
+    tm.assert_frame_equal(df2, df2_orig)
+
+
+def test_merge_on_index(using_copy_on_write):
+    df1 = DataFrame({"a": [1, 2, 3]})
+    df2 = DataFrame({"b": [4, 5, 6]})
+    df1_orig = df1.copy()
+    df2_orig = df2.copy()
+
+    result = merge(df1, df2, left_index=True, right_index=True)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    result.iloc[0, 0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    result.iloc[0, 1] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+    tm.assert_frame_equal(df1, df1_orig)
+    tm.assert_frame_equal(df2, df2_orig)
+
+
+@pytest.mark.parametrize(
+    "func, how",
+    [
+        (lambda df1, df2, **kwargs: merge(df2, df1, on="key", **kwargs), "right"),
+        (lambda df1, df2, **kwargs: merge(df1, df2, on="key", **kwargs), "left"),
+    ],
+)
+def test_merge_on_key_enlarging_one(using_copy_on_write, func, how):
+    df1 = DataFrame({"key": ["a", "b", "c"], "a": [1, 2, 3]})
+    df2 = DataFrame({"key": ["a", "b"], "b": [4, 5]})
+    df1_orig = df1.copy()
+    df2_orig = df2.copy()
+
+    result = func(df1, df2, how=how)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+        assert df2._mgr._has_no_reference(1)
+        assert df2._mgr._has_no_reference(0)
+        assert np.shares_memory(get_array(result, "key"), get_array(df1, "key")) is (
+            how == "left"
+        )
+        assert not np.shares_memory(get_array(result, "key"), get_array(df2, "key"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    if how == "left":
+        result.iloc[0, 1] = 0
+    else:
+        result.iloc[0, 2] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+    tm.assert_frame_equal(df1, df1_orig)
+    tm.assert_frame_equal(df2, df2_orig)
+
+
+@pytest.mark.parametrize("copy", [True, None, False])
+def test_merge_copy_keyword(using_copy_on_write, copy):
+    df = DataFrame({"a": [1, 2]})
+    df2 = DataFrame({"b": [3, 4.5]})
+
+    result = df.merge(df2, copy=copy, left_index=True, right_index=True)
+
+    if using_copy_on_write or copy is False:
+        assert np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+        assert np.shares_memory(get_array(df2, "b"), get_array(result, "b"))
+    else:
+        assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+        assert not np.shares_memory(get_array(df2, "b"), get_array(result, "b"))
+
+
+def test_join_on_key(using_copy_on_write):
+    df_index = Index(["a", "b", "c"], name="key")
+
+    df1 = DataFrame({"a": [1, 2, 3]}, index=df_index.copy(deep=True))
+    df2 = DataFrame({"b": [4, 5, 6]}, index=df_index.copy(deep=True))
+
+    df1_orig = df1.copy()
+    df2_orig = df2.copy()
+
+    result = df1.join(df2, on="key")
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+        assert np.shares_memory(get_array(result.index), get_array(df1.index))
+        assert not np.shares_memory(get_array(result.index), get_array(df2.index))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    result.iloc[0, 0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    result.iloc[0, 1] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(df2, "b"))
+
+    tm.assert_frame_equal(df1, df1_orig)
+    tm.assert_frame_equal(df2, df2_orig)
+
+
+def test_join_multiple_dataframes_on_key(using_copy_on_write):
+    df_index = Index(["a", "b", "c"], name="key")
+
+    df1 = DataFrame({"a": [1, 2, 3]}, index=df_index.copy(deep=True))
+    dfs_list = [
+        DataFrame({"b": [4, 5, 6]}, index=df_index.copy(deep=True)),
+        DataFrame({"c": [7, 8, 9]}, index=df_index.copy(deep=True)),
+    ]
+
+    df1_orig = df1.copy()
+    dfs_list_orig = [df.copy() for df in dfs_list]
+
+    result = df1.join(dfs_list)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(dfs_list[0], "b"))
+        assert np.shares_memory(get_array(result, "c"), get_array(dfs_list[1], "c"))
+        assert np.shares_memory(get_array(result.index), get_array(df1.index))
+        assert not np.shares_memory(
+            get_array(result.index), get_array(dfs_list[0].index)
+        )
+        assert not np.shares_memory(
+            get_array(result.index), get_array(dfs_list[1].index)
+        )
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert not np.shares_memory(get_array(result, "b"), get_array(dfs_list[0], "b"))
+        assert not np.shares_memory(get_array(result, "c"), get_array(dfs_list[1], "c"))
+
+    result.iloc[0, 0] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df1, "a"))
+        assert np.shares_memory(get_array(result, "b"), get_array(dfs_list[0], "b"))
+        assert np.shares_memory(get_array(result, "c"), get_array(dfs_list[1], "c"))
+
+    result.iloc[0, 1] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "b"), get_array(dfs_list[0], "b"))
+        assert np.shares_memory(get_array(result, "c"), get_array(dfs_list[1], "c"))
+
+    result.iloc[0, 2] = 0
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "c"), get_array(dfs_list[1], "c"))
+
+    tm.assert_frame_equal(df1, df1_orig)
+    for df, df_orig in zip(dfs_list, dfs_list_orig):
+        tm.assert_frame_equal(df, df_orig)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_internals.py

@@ -0,0 +1,151 @@
+import numpy as np
+import pytest
+
+import pandas.util._test_decorators as td
+
+import pandas as pd
+from pandas import DataFrame
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+@td.skip_array_manager_invalid_test
+def test_consolidate(using_copy_on_write):
+    # create unconsolidated DataFrame
+    df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+    df["c"] = [4, 5, 6]
+
+    # take a viewing subset
+    subset = df[:]
+
+    # each block of subset references a block of df
+    assert all(blk.refs.has_reference() for blk in subset._mgr.blocks)
+
+    # consolidate the two int64 blocks
+    subset._consolidate_inplace()
+
+    # the float64 block still references the parent one because it still a view
+    assert subset._mgr.blocks[0].refs.has_reference()
+    # equivalent of assert np.shares_memory(df["b"].values, subset["b"].values)
+    # but avoids caching df["b"]
+    assert np.shares_memory(get_array(df, "b"), get_array(subset, "b"))
+
+    # the new consolidated int64 block does not reference another
+    assert not subset._mgr.blocks[1].refs.has_reference()
+
+    # the parent dataframe now also only is linked for the float column
+    assert not df._mgr.blocks[0].refs.has_reference()
+    assert df._mgr.blocks[1].refs.has_reference()
+    assert not df._mgr.blocks[2].refs.has_reference()
+
+    # and modifying subset still doesn't modify parent
+    if using_copy_on_write:
+        subset.iloc[0, 1] = 0.0
+        assert not df._mgr.blocks[1].refs.has_reference()
+        assert df.loc[0, "b"] == 0.1
+
+
+@pytest.mark.single_cpu
+@td.skip_array_manager_invalid_test
+def test_switch_options():
+    # ensure we can switch the value of the option within one session
+    # (assuming data is constructed after switching)
+
+    # using the option_context to ensure we set back to global option value
+    # after running the test
+    with pd.option_context("mode.copy_on_write", False):
+        df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+        subset = df[:]
+        subset.iloc[0, 0] = 0
+        # df updated with CoW disabled
+        assert df.iloc[0, 0] == 0
+
+        pd.options.mode.copy_on_write = True
+        df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+        subset = df[:]
+        subset.iloc[0, 0] = 0
+        # df not updated with CoW enabled
+        assert df.iloc[0, 0] == 1
+
+        pd.options.mode.copy_on_write = False
+        df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+        subset = df[:]
+        subset.iloc[0, 0] = 0
+        # df updated with CoW disabled
+        assert df.iloc[0, 0] == 0
+
+
+@td.skip_array_manager_invalid_test
+@pytest.mark.parametrize("dtype", [np.intp, np.int8])
+@pytest.mark.parametrize(
+    "locs, arr",
+    [
+        ([0], np.array([-1, -2, -3])),
+        ([1], np.array([-1, -2, -3])),
+        ([5], np.array([-1, -2, -3])),
+        ([0, 1], np.array([[-1, -2, -3], [-4, -5, -6]]).T),
+        ([0, 2], np.array([[-1, -2, -3], [-4, -5, -6]]).T),
+        ([0, 1, 2], np.array([[-1, -2, -3], [-4, -5, -6], [-4, -5, -6]]).T),
+        ([1, 2], np.array([[-1, -2, -3], [-4, -5, -6]]).T),
+        ([1, 3], np.array([[-1, -2, -3], [-4, -5, -6]]).T),
+        ([1, 3], np.array([[-1, -2, -3], [-4, -5, -6]]).T),
+    ],
+)
+def test_iset_splits_blocks_inplace(using_copy_on_write, locs, arr, dtype):
+    # Nothing currently calls iset with
+    # more than 1 loc with inplace=True (only happens with inplace=False)
+    # but ensure that it works
+    df = DataFrame(
+        {
+            "a": [1, 2, 3],
+            "b": [4, 5, 6],
+            "c": [7, 8, 9],
+            "d": [10, 11, 12],
+            "e": [13, 14, 15],
+            "f": ["a", "b", "c"],
+        },
+    )
+    arr = arr.astype(dtype)
+    df_orig = df.copy()
+    df2 = df.copy(deep=None)  # Trigger a CoW (if enabled, otherwise makes copy)
+    df2._mgr.iset(locs, arr, inplace=True)
+
+    tm.assert_frame_equal(df, df_orig)
+
+    if using_copy_on_write:
+        for i, col in enumerate(df.columns):
+            if i not in locs:
+                assert np.shares_memory(get_array(df, col), get_array(df2, col))
+    else:
+        for col in df.columns:
+            assert not np.shares_memory(get_array(df, col), get_array(df2, col))
+
+
+def test_exponential_backoff():
+    # GH#55518
+    df = DataFrame({"a": [1, 2, 3]})
+    for i in range(490):
+        df.copy(deep=False)
+
+    assert len(df._mgr.blocks[0].refs.referenced_blocks) == 491
+
+    df = DataFrame({"a": [1, 2, 3]})
+    dfs = [df.copy(deep=False) for i in range(510)]
+
+    for i in range(20):
+        df.copy(deep=False)
+    assert len(df._mgr.blocks[0].refs.referenced_blocks) == 531
+    assert df._mgr.blocks[0].refs.clear_counter == 1000
+
+    for i in range(500):
+        df.copy(deep=False)
+
+    # Don't reduce since we still have over 500 objects alive
+    assert df._mgr.blocks[0].refs.clear_counter == 1000
+
+    dfs = dfs[:300]
+    for i in range(500):
+        df.copy(deep=False)
+
+    # Reduce since there are less than 500 objects alive
+    assert df._mgr.blocks[0].refs.clear_counter == 500

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_interp_fillna.py

@@ -0,0 +1,432 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    NA,
+    ArrowDtype,
+    DataFrame,
+    Interval,
+    NaT,
+    Series,
+    Timestamp,
+    interval_range,
+    option_context,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+@pytest.mark.parametrize("method", ["pad", "nearest", "linear"])
+def test_interpolate_no_op(using_copy_on_write, method):
+    df = DataFrame({"a": [1, 2]})
+    df_orig = df.copy()
+
+    warn = None
+    if method == "pad":
+        warn = FutureWarning
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(warn, match=msg):
+        result = df.interpolate(method=method)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+
+    result.iloc[0, 0] = 100
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+@pytest.mark.parametrize("func", ["ffill", "bfill"])
+def test_interp_fill_functions(using_copy_on_write, func):
+    # Check that these takes the same code paths as interpolate
+    df = DataFrame({"a": [1, 2]})
+    df_orig = df.copy()
+
+    result = getattr(df, func)()
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+
+    result.iloc[0, 0] = 100
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+@pytest.mark.parametrize("func", ["ffill", "bfill"])
+@pytest.mark.parametrize(
+    "vals", [[1, np.nan, 2], [Timestamp("2019-12-31"), NaT, Timestamp("2020-12-31")]]
+)
+def test_interpolate_triggers_copy(using_copy_on_write, vals, func):
+    df = DataFrame({"a": vals})
+    result = getattr(df, func)()
+
+    assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    if using_copy_on_write:
+        # Check that we don't have references when triggering a copy
+        assert result._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize(
+    "vals", [[1, np.nan, 2], [Timestamp("2019-12-31"), NaT, Timestamp("2020-12-31")]]
+)
+def test_interpolate_inplace_no_reference_no_copy(using_copy_on_write, vals):
+    df = DataFrame({"a": vals})
+    arr = get_array(df, "a")
+    df.interpolate(method="linear", inplace=True)
+
+    assert np.shares_memory(arr, get_array(df, "a"))
+    if using_copy_on_write:
+        # Check that we don't have references when triggering a copy
+        assert df._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize(
+    "vals", [[1, np.nan, 2], [Timestamp("2019-12-31"), NaT, Timestamp("2020-12-31")]]
+)
+def test_interpolate_inplace_with_refs(using_copy_on_write, vals, warn_copy_on_write):
+    df = DataFrame({"a": [1, np.nan, 2]})
+    df_orig = df.copy()
+    arr = get_array(df, "a")
+    view = df[:]
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.interpolate(method="linear", inplace=True)
+
+    if using_copy_on_write:
+        # Check that copy was triggered in interpolate and that we don't
+        # have any references left
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        tm.assert_frame_equal(df_orig, view)
+        assert df._mgr._has_no_reference(0)
+        assert view._mgr._has_no_reference(0)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a"))
+
+
+@pytest.mark.parametrize("func", ["ffill", "bfill"])
+@pytest.mark.parametrize("dtype", ["float64", "Float64"])
+def test_interp_fill_functions_inplace(
+    using_copy_on_write, func, warn_copy_on_write, dtype
+):
+    # Check that these takes the same code paths as interpolate
+    df = DataFrame({"a": [1, np.nan, 2]}, dtype=dtype)
+    df_orig = df.copy()
+    arr = get_array(df, "a")
+    view = df[:]
+
+    with tm.assert_cow_warning(warn_copy_on_write and dtype == "float64"):
+        getattr(df, func)(inplace=True)
+
+    if using_copy_on_write:
+        # Check that copy was triggered in interpolate and that we don't
+        # have any references left
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        tm.assert_frame_equal(df_orig, view)
+        assert df._mgr._has_no_reference(0)
+        assert view._mgr._has_no_reference(0)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a")) is (dtype == "float64")
+
+
+def test_interpolate_cleaned_fill_method(using_copy_on_write):
+    # Check that "method is set to None" case works correctly
+    df = DataFrame({"a": ["a", np.nan, "c"], "b": 1})
+    df_orig = df.copy()
+
+    msg = "DataFrame.interpolate with object dtype"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        result = df.interpolate(method="linear")
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+
+    result.iloc[0, 0] = Timestamp("2021-12-31")
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_interpolate_object_convert_no_op(using_copy_on_write):
+    df = DataFrame({"a": ["a", "b", "c"], "b": 1})
+    arr_a = get_array(df, "a")
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True)
+
+    # Now CoW makes a copy, it should not!
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert np.shares_memory(arr_a, get_array(df, "a"))
+
+
+def test_interpolate_object_convert_copies(using_copy_on_write):
+    df = DataFrame({"a": Series([1, 2], dtype=object), "b": 1})
+    arr_a = get_array(df, "a")
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True)
+
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert not np.shares_memory(arr_a, get_array(df, "a"))
+
+
+def test_interpolate_downcast(using_copy_on_write):
+    df = DataFrame({"a": [1, np.nan, 2.5], "b": 1})
+    arr_a = get_array(df, "a")
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True, downcast="infer")
+
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+    assert np.shares_memory(arr_a, get_array(df, "a"))
+
+
+def test_interpolate_downcast_reference_triggers_copy(using_copy_on_write):
+    df = DataFrame({"a": [1, np.nan, 2.5], "b": 1})
+    df_orig = df.copy()
+    arr_a = get_array(df, "a")
+    view = df[:]
+    msg = "DataFrame.interpolate with method=pad is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.interpolate(method="pad", inplace=True, downcast="infer")
+
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert not np.shares_memory(arr_a, get_array(df, "a"))
+        tm.assert_frame_equal(df_orig, view)
+    else:
+        tm.assert_frame_equal(df, view)
+
+
+def test_fillna(using_copy_on_write):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    df_orig = df.copy()
+
+    df2 = df.fillna(5.5)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+    else:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+
+    df2.iloc[0, 1] = 100
+    tm.assert_frame_equal(df_orig, df)
+
+
+def test_fillna_dict(using_copy_on_write):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    df_orig = df.copy()
+
+    df2 = df.fillna({"a": 100.5})
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+    else:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+
+    df2.iloc[0, 1] = 100
+    tm.assert_frame_equal(df_orig, df)
+
+
+@pytest.mark.parametrize("downcast", [None, False])
+def test_fillna_inplace(using_copy_on_write, downcast):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    arr_a = get_array(df, "a")
+    arr_b = get_array(df, "b")
+
+    msg = "The 'downcast' keyword in fillna is deprecated"
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.fillna(5.5, inplace=True, downcast=downcast)
+    assert np.shares_memory(get_array(df, "a"), arr_a)
+    assert np.shares_memory(get_array(df, "b"), arr_b)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert df._mgr._has_no_reference(1)
+
+
+def test_fillna_inplace_reference(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1.5, np.nan], "b": 1})
+    df_orig = df.copy()
+    arr_a = get_array(df, "a")
+    arr_b = get_array(df, "b")
+    view = df[:]
+
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.fillna(5.5, inplace=True)
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr_a)
+        assert np.shares_memory(get_array(df, "b"), arr_b)
+        assert view._mgr._has_no_reference(0)
+        assert df._mgr._has_no_reference(0)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr_a)
+        assert np.shares_memory(get_array(df, "b"), arr_b)
+    expected = DataFrame({"a": [1.5, 5.5], "b": 1})
+    tm.assert_frame_equal(df, expected)
+
+
+def test_fillna_interval_inplace_reference(using_copy_on_write, warn_copy_on_write):
+    # Set dtype explicitly to avoid implicit cast when setting nan
+    ser = Series(
+        interval_range(start=0, end=5), name="a", dtype="interval[float64, right]"
+    )
+    ser.iloc[1] = np.nan
+
+    ser_orig = ser.copy()
+    view = ser[:]
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.fillna(value=Interval(left=0, right=5), inplace=True)
+
+    if using_copy_on_write:
+        assert not np.shares_memory(
+            get_array(ser, "a").left.values, get_array(view, "a").left.values
+        )
+        tm.assert_series_equal(view, ser_orig)
+    else:
+        assert np.shares_memory(
+            get_array(ser, "a").left.values, get_array(view, "a").left.values
+        )
+
+
+def test_fillna_series_empty_arg(using_copy_on_write):
+    ser = Series([1, np.nan, 2])
+    ser_orig = ser.copy()
+    result = ser.fillna({})
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(ser), get_array(result))
+    else:
+        assert not np.shares_memory(get_array(ser), get_array(result))
+
+    ser.iloc[0] = 100.5
+    tm.assert_series_equal(ser_orig, result)
+
+
+def test_fillna_series_empty_arg_inplace(using_copy_on_write):
+    ser = Series([1, np.nan, 2])
+    arr = get_array(ser)
+    ser.fillna({}, inplace=True)
+
+    assert np.shares_memory(get_array(ser), arr)
+    if using_copy_on_write:
+        assert ser._mgr._has_no_reference(0)
+
+
+def test_fillna_ea_noop_shares_memory(
+    using_copy_on_write, any_numeric_ea_and_arrow_dtype
+):
+    df = DataFrame({"a": [1, NA, 3], "b": 1}, dtype=any_numeric_ea_and_arrow_dtype)
+    df_orig = df.copy()
+    df2 = df.fillna(100)
+
+    assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert not df2._mgr._has_no_reference(1)
+    elif isinstance(df.dtypes.iloc[0], ArrowDtype):
+        # arrow is immutable, so no-ops do not need to copy underlying array
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+    else:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+
+    tm.assert_frame_equal(df_orig, df)
+
+    df2.iloc[0, 1] = 100
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert df2._mgr._has_no_reference(1)
+        assert df._mgr._has_no_reference(1)
+    tm.assert_frame_equal(df_orig, df)
+
+
+def test_fillna_inplace_ea_noop_shares_memory(
+    using_copy_on_write, warn_copy_on_write, any_numeric_ea_and_arrow_dtype
+):
+    df = DataFrame({"a": [1, NA, 3], "b": 1}, dtype=any_numeric_ea_and_arrow_dtype)
+    df_orig = df.copy()
+    view = df[:]
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.fillna(100, inplace=True)
+
+    if isinstance(df["a"].dtype, ArrowDtype) or using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), get_array(view, "a"))
+    else:
+        # MaskedArray can actually respect inplace=True
+        assert np.shares_memory(get_array(df, "a"), get_array(view, "a"))
+
+    assert np.shares_memory(get_array(df, "b"), get_array(view, "b"))
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(1)
+        assert not view._mgr._has_no_reference(1)
+
+    with tm.assert_cow_warning(
+        warn_copy_on_write and "pyarrow" not in any_numeric_ea_and_arrow_dtype
+    ):
+        df.iloc[0, 1] = 100
+    if isinstance(df["a"].dtype, ArrowDtype) or using_copy_on_write:
+        tm.assert_frame_equal(df_orig, view)
+    else:
+        # we actually have a view
+        tm.assert_frame_equal(df, view)
+
+
+def test_fillna_chained_assignment(using_copy_on_write):
+    df = DataFrame({"a": [1, np.nan, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].fillna(100, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            df[["a"]].fillna(100, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                df[["a"]].fillna(100, inplace=True)
+
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                df[df.a > 5].fillna(100, inplace=True)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].fillna(100, inplace=True)
+
+
+@pytest.mark.parametrize("func", ["interpolate", "ffill", "bfill"])
+def test_interpolate_chained_assignment(using_copy_on_write, func):
+    df = DataFrame({"a": [1, np.nan, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            getattr(df["a"], func)(inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            getattr(df[["a"]], func)(inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            getattr(df["a"], func)(inplace=True)
+
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                getattr(df[["a"]], func)(inplace=True)
+
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                getattr(df[df["a"] > 1], func)(inplace=True)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_replace.py

@@ -0,0 +1,481 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    Categorical,
+    DataFrame,
+    option_context,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+
+@pytest.mark.parametrize(
+    "replace_kwargs",
+    [
+        {"to_replace": {"a": 1, "b": 4}, "value": -1},
+        # Test CoW splits blocks to avoid copying unchanged columns
+        {"to_replace": {"a": 1}, "value": -1},
+        {"to_replace": {"b": 4}, "value": -1},
+        {"to_replace": {"b": {4: 1}}},
+        # TODO: Add these in a further optimization
+        # We would need to see which columns got replaced in the mask
+        # which could be expensive
+        # {"to_replace": {"b": 1}},
+        # 1
+    ],
+)
+def test_replace(using_copy_on_write, replace_kwargs):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": ["foo", "bar", "baz"]})
+    df_orig = df.copy()
+
+    df_replaced = df.replace(**replace_kwargs)
+
+    if using_copy_on_write:
+        if (df_replaced["b"] == df["b"]).all():
+            assert np.shares_memory(get_array(df_replaced, "b"), get_array(df, "b"))
+        assert np.shares_memory(get_array(df_replaced, "c"), get_array(df, "c"))
+
+    # mutating squeezed df triggers a copy-on-write for that column/block
+    df_replaced.loc[0, "c"] = -1
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df_replaced, "c"), get_array(df, "c"))
+
+    if "a" in replace_kwargs["to_replace"]:
+        arr = get_array(df_replaced, "a")
+        df_replaced.loc[0, "a"] = 100
+        assert np.shares_memory(get_array(df_replaced, "a"), arr)
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_replace_regex_inplace_refs(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": ["aaa", "bbb"]})
+    df_orig = df.copy()
+    view = df[:]
+    arr = get_array(df, "a")
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace(to_replace=r"^a.*$", value="new", inplace=True, regex=True)
+    if using_copy_on_write:
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        assert df._mgr._has_no_reference(0)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_replace_regex_inplace(using_copy_on_write):
+    df = DataFrame({"a": ["aaa", "bbb"]})
+    arr = get_array(df, "a")
+    df.replace(to_replace=r"^a.*$", value="new", inplace=True, regex=True)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+    assert np.shares_memory(arr, get_array(df, "a"))
+
+    df_orig = df.copy()
+    df2 = df.replace(to_replace=r"^b.*$", value="new", regex=True)
+    tm.assert_frame_equal(df_orig, df)
+    assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+
+def test_replace_regex_inplace_no_op(using_copy_on_write):
+    df = DataFrame({"a": [1, 2]})
+    arr = get_array(df, "a")
+    df.replace(to_replace=r"^a.$", value="new", inplace=True, regex=True)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+    assert np.shares_memory(arr, get_array(df, "a"))
+
+    df_orig = df.copy()
+    df2 = df.replace(to_replace=r"^x.$", value="new", regex=True)
+    tm.assert_frame_equal(df_orig, df)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+
+def test_replace_mask_all_false_second_block(using_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3], "b": 100.5, "c": 1, "d": 2})
+    df_orig = df.copy()
+
+    df2 = df.replace(to_replace=1.5, value=55.5)
+
+    if using_copy_on_write:
+        # TODO: Block splitting would allow us to avoid copying b
+        assert np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    else:
+        assert not np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    df2.loc[0, "c"] = 1
+    tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+        # TODO: This should split and not copy the whole block
+        # assert np.shares_memory(get_array(df, "d"), get_array(df2, "d"))
+
+
+def test_replace_coerce_single_column(using_copy_on_write, using_array_manager):
+    df = DataFrame({"a": [1.5, 2, 3], "b": 100.5})
+    df_orig = df.copy()
+
+    df2 = df.replace(to_replace=1.5, value="a")
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    elif not using_array_manager:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    if using_copy_on_write:
+        df2.loc[0, "b"] = 0.5
+        tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+
+
+def test_replace_to_replace_wrong_dtype(using_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3], "b": 100.5})
+    df_orig = df.copy()
+
+    df2 = df.replace(to_replace="xxx", value=1.5)
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    else:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+        assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    df2.loc[0, "b"] = 0.5
+    tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+
+
+def test_replace_list_categorical(using_copy_on_write):
+    df = DataFrame({"a": ["a", "b", "c"]}, dtype="category")
+    arr = get_array(df, "a")
+    msg = (
+        r"The behavior of Series\.replace \(and DataFrame.replace\) "
+        "with CategoricalDtype"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.replace(["c"], value="a", inplace=True)
+    assert np.shares_memory(arr.codes, get_array(df, "a").codes)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+    df_orig = df.copy()
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df2 = df.replace(["b"], value="a")
+    assert not np.shares_memory(arr.codes, get_array(df2, "a").codes)
+
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_replace_list_inplace_refs_categorical(using_copy_on_write):
+    df = DataFrame({"a": ["a", "b", "c"]}, dtype="category")
+    view = df[:]
+    df_orig = df.copy()
+    msg = (
+        r"The behavior of Series\.replace \(and DataFrame.replace\) "
+        "with CategoricalDtype"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        df.replace(["c"], value="a", inplace=True)
+    if using_copy_on_write:
+        assert not np.shares_memory(
+            get_array(view, "a").codes, get_array(df, "a").codes
+        )
+        tm.assert_frame_equal(df_orig, view)
+    else:
+        # This could be inplace
+        assert not np.shares_memory(
+            get_array(view, "a").codes, get_array(df, "a").codes
+        )
+
+
+@pytest.mark.parametrize("to_replace", [1.5, [1.5], []])
+def test_replace_inplace(using_copy_on_write, to_replace):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    arr_a = get_array(df, "a")
+    df.replace(to_replace=1.5, value=15.5, inplace=True)
+
+    assert np.shares_memory(get_array(df, "a"), arr_a)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize("to_replace", [1.5, [1.5]])
+def test_replace_inplace_reference(using_copy_on_write, to_replace, warn_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    arr_a = get_array(df, "a")
+    view = df[:]
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace(to_replace=to_replace, value=15.5, inplace=True)
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr_a)
+        assert df._mgr._has_no_reference(0)
+        assert view._mgr._has_no_reference(0)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr_a)
+
+
+@pytest.mark.parametrize("to_replace", ["a", 100.5])
+def test_replace_inplace_reference_no_op(using_copy_on_write, to_replace):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    arr_a = get_array(df, "a")
+    view = df[:]
+    df.replace(to_replace=to_replace, value=15.5, inplace=True)
+
+    assert np.shares_memory(get_array(df, "a"), arr_a)
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+        assert not view._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize("to_replace", [1, [1]])
+@pytest.mark.parametrize("val", [1, 1.5])
+def test_replace_categorical_inplace_reference(using_copy_on_write, val, to_replace):
+    df = DataFrame({"a": Categorical([1, 2, 3])})
+    df_orig = df.copy()
+    arr_a = get_array(df, "a")
+    view = df[:]
+    msg = (
+        r"The behavior of Series\.replace \(and DataFrame.replace\) "
+        "with CategoricalDtype"
+    )
+    warn = FutureWarning if val == 1.5 else None
+    with tm.assert_produces_warning(warn, match=msg):
+        df.replace(to_replace=to_replace, value=val, inplace=True)
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a").codes, arr_a.codes)
+        assert df._mgr._has_no_reference(0)
+        assert view._mgr._has_no_reference(0)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a").codes, arr_a.codes)
+
+
+@pytest.mark.parametrize("val", [1, 1.5])
+def test_replace_categorical_inplace(using_copy_on_write, val):
+    df = DataFrame({"a": Categorical([1, 2, 3])})
+    arr_a = get_array(df, "a")
+    msg = (
+        r"The behavior of Series\.replace \(and DataFrame.replace\) "
+        "with CategoricalDtype"
+    )
+    warn = FutureWarning if val == 1.5 else None
+    with tm.assert_produces_warning(warn, match=msg):
+        df.replace(to_replace=1, value=val, inplace=True)
+
+    assert np.shares_memory(get_array(df, "a").codes, arr_a.codes)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+    expected = DataFrame({"a": Categorical([val, 2, 3])})
+    tm.assert_frame_equal(df, expected)
+
+
+@pytest.mark.parametrize("val", [1, 1.5])
+def test_replace_categorical(using_copy_on_write, val):
+    df = DataFrame({"a": Categorical([1, 2, 3])})
+    df_orig = df.copy()
+    msg = (
+        r"The behavior of Series\.replace \(and DataFrame.replace\) "
+        "with CategoricalDtype"
+    )
+    warn = FutureWarning if val == 1.5 else None
+    with tm.assert_produces_warning(warn, match=msg):
+        df2 = df.replace(to_replace=1, value=val)
+
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert df2._mgr._has_no_reference(0)
+    assert not np.shares_memory(get_array(df, "a").codes, get_array(df2, "a").codes)
+    tm.assert_frame_equal(df, df_orig)
+
+    arr_a = get_array(df2, "a").codes
+    df2.iloc[0, 0] = 2.0
+    assert np.shares_memory(get_array(df2, "a").codes, arr_a)
+
+
+@pytest.mark.parametrize("method", ["where", "mask"])
+def test_masking_inplace(using_copy_on_write, method, warn_copy_on_write):
+    df = DataFrame({"a": [1.5, 2, 3]})
+    df_orig = df.copy()
+    arr_a = get_array(df, "a")
+    view = df[:]
+
+    method = getattr(df, method)
+    if warn_copy_on_write:
+        with tm.assert_cow_warning():
+            method(df["a"] > 1.6, -1, inplace=True)
+    else:
+        method(df["a"] > 1.6, -1, inplace=True)
+
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr_a)
+        assert df._mgr._has_no_reference(0)
+        assert view._mgr._has_no_reference(0)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr_a)
+
+
+def test_replace_empty_list(using_copy_on_write):
+    df = DataFrame({"a": [1, 2]})
+
+    df2 = df.replace([], [])
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+        assert not df._mgr._has_no_reference(0)
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    arr_a = get_array(df, "a")
+    df.replace([], [])
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "a"), arr_a)
+        assert not df._mgr._has_no_reference(0)
+        assert not df2._mgr._has_no_reference(0)
+
+
+@pytest.mark.parametrize("value", ["d", None])
+def test_replace_object_list_inplace(using_copy_on_write, value):
+    df = DataFrame({"a": ["a", "b", "c"]})
+    arr = get_array(df, "a")
+    df.replace(["c"], value, inplace=True)
+    if using_copy_on_write or value is None:
+        assert np.shares_memory(arr, get_array(df, "a"))
+    else:
+        # This could be inplace
+        assert not np.shares_memory(arr, get_array(df, "a"))
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+def test_replace_list_multiple_elements_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]})
+    arr = get_array(df, "a")
+    df.replace([1, 2], 4, inplace=True)
+    if using_copy_on_write:
+        assert np.shares_memory(arr, get_array(df, "a"))
+        assert df._mgr._has_no_reference(0)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_replace_list_none(using_copy_on_write):
+    df = DataFrame({"a": ["a", "b", "c"]})
+
+    df_orig = df.copy()
+    df2 = df.replace(["b"], value=None)
+    tm.assert_frame_equal(df, df_orig)
+
+    assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+
+def test_replace_list_none_inplace_refs(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": ["a", "b", "c"]})
+    arr = get_array(df, "a")
+    df_orig = df.copy()
+    view = df[:]
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace(["a"], value=None, inplace=True)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        tm.assert_frame_equal(df_orig, view)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_replace_columnwise_no_op_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [1, 2, 3]})
+    view = df[:]
+    df_orig = df.copy()
+    df.replace({"a": 10}, 100, inplace=True)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(view, "a"), get_array(df, "a"))
+        df.iloc[0, 0] = 100
+        tm.assert_frame_equal(view, df_orig)
+
+
+def test_replace_columnwise_no_op(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [1, 2, 3]})
+    df_orig = df.copy()
+    df2 = df.replace({"a": 10}, 100)
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    df2.iloc[0, 0] = 100
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_replace_chained_assignment(using_copy_on_write):
+    df = DataFrame({"a": [1, np.nan, 2], "b": 1})
+    df_orig = df.copy()
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].replace(1, 100, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+
+        with tm.raises_chained_assignment_error():
+            df[["a"]].replace(1, 100, inplace=True)
+        tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                df[["a"]].replace(1, 100, inplace=True)
+
+        with tm.assert_produces_warning(None):
+            with option_context("mode.chained_assignment", None):
+                df[df.a > 5].replace(1, 100, inplace=True)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].replace(1, 100, inplace=True)
+
+
+def test_replace_listlike(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [1, 2, 3]})
+    df_orig = df.copy()
+
+    result = df.replace([200, 201], [11, 11])
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+
+    result.iloc[0, 0] = 100
+    tm.assert_frame_equal(df, df)
+
+    result = df.replace([200, 2], [10, 10])
+    assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_replace_listlike_inplace(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [1, 2, 3]})
+    arr = get_array(df, "a")
+    df.replace([200, 2], [10, 11], inplace=True)
+    assert np.shares_memory(get_array(df, "a"), arr)
+
+    view = df[:]
+    df_orig = df.copy()
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace([200, 3], [10, 11], inplace=True)
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr)
+        tm.assert_frame_equal(df, view)

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_setitem.py

@@ -0,0 +1,156 @@
+import numpy as np
+
+from pandas import (
+    DataFrame,
+    Index,
+    MultiIndex,
+    RangeIndex,
+    Series,
+)
+import pandas._testing as tm
+from pandas.tests.copy_view.util import get_array
+
+# -----------------------------------------------------------------------------
+# Copy/view behaviour for the values that are set in a DataFrame
+
+
+def test_set_column_with_array():
+    # Case: setting an array as a new column (df[col] = arr) copies that data
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    arr = np.array([1, 2, 3], dtype="int64")
+
+    df["c"] = arr
+
+    # the array data is copied
+    assert not np.shares_memory(get_array(df, "c"), arr)
+    # and thus modifying the array does not modify the DataFrame
+    arr[0] = 0
+    tm.assert_series_equal(df["c"], Series([1, 2, 3], name="c"))
+
+
+def test_set_column_with_series(using_copy_on_write):
+    # Case: setting a series as a new column (df[col] = s) copies that data
+    # (with delayed copy with CoW)
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    ser = Series([1, 2, 3])
+
+    df["c"] = ser
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "c"), get_array(ser))
+    else:
+        # the series data is copied
+        assert not np.shares_memory(get_array(df, "c"), get_array(ser))
+
+    # and modifying the series does not modify the DataFrame
+    ser.iloc[0] = 0
+    assert ser.iloc[0] == 0
+    tm.assert_series_equal(df["c"], Series([1, 2, 3], name="c"))
+
+
+def test_set_column_with_index(using_copy_on_write):
+    # Case: setting an index as a new column (df[col] = idx) copies that data
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    idx = Index([1, 2, 3])
+
+    df["c"] = idx
+
+    # the index data is copied
+    assert not np.shares_memory(get_array(df, "c"), idx.values)
+
+    idx = RangeIndex(1, 4)
+    arr = idx.values
+
+    df["d"] = idx
+
+    assert not np.shares_memory(get_array(df, "d"), arr)
+
+
+def test_set_columns_with_dataframe(using_copy_on_write):
+    # Case: setting a DataFrame as new columns copies that data
+    # (with delayed copy with CoW)
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    df2 = DataFrame({"c": [7, 8, 9], "d": [10, 11, 12]})
+
+    df[["c", "d"]] = df2
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+    else:
+        # the data is copied
+        assert not np.shares_memory(get_array(df, "c"), get_array(df2, "c"))
+
+    # and modifying the set DataFrame does not modify the original DataFrame
+    df2.iloc[0, 0] = 0
+    tm.assert_series_equal(df["c"], Series([7, 8, 9], name="c"))
+
+
+def test_setitem_series_no_copy(using_copy_on_write):
+    # Case: setting a Series as column into a DataFrame can delay copying that data
+    df = DataFrame({"a": [1, 2, 3]})
+    rhs = Series([4, 5, 6])
+    rhs_orig = rhs.copy()
+
+    # adding a new column
+    df["b"] = rhs
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(rhs), get_array(df, "b"))
+
+    df.iloc[0, 1] = 100
+    tm.assert_series_equal(rhs, rhs_orig)
+
+
+def test_setitem_series_no_copy_single_block(using_copy_on_write):
+    # Overwriting an existing column that is a single block
+    df = DataFrame({"a": [1, 2, 3], "b": [0.1, 0.2, 0.3]})
+    rhs = Series([4, 5, 6])
+    rhs_orig = rhs.copy()
+
+    df["a"] = rhs
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(rhs), get_array(df, "a"))
+
+    df.iloc[0, 0] = 100
+    tm.assert_series_equal(rhs, rhs_orig)
+
+
+def test_setitem_series_no_copy_split_block(using_copy_on_write):
+    # Overwriting an existing column that is part of a larger block
+    df = DataFrame({"a": [1, 2, 3], "b": 1})
+    rhs = Series([4, 5, 6])
+    rhs_orig = rhs.copy()
+
+    df["b"] = rhs
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(rhs), get_array(df, "b"))
+
+    df.iloc[0, 1] = 100
+    tm.assert_series_equal(rhs, rhs_orig)
+
+
+def test_setitem_series_column_midx_broadcasting(using_copy_on_write):
+    # Setting a Series to multiple columns will repeat the data
+    # (currently copying the data eagerly)
+    df = DataFrame(
+        [[1, 2, 3], [3, 4, 5]],
+        columns=MultiIndex.from_arrays([["a", "a", "b"], [1, 2, 3]]),
+    )
+    rhs = Series([10, 11])
+    df["a"] = rhs
+    assert not np.shares_memory(get_array(rhs), df._get_column_array(0))
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+def test_set_column_with_inplace_operator(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+
+    # this should not raise any warning
+    with tm.assert_produces_warning(None):
+        df["a"] += 1
+
+    # when it is not in a chain, then it should produce a warning
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    ser = df["a"]
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser += 1

venv/lib/python3.12/site-packages/pandas/tests/copy_view/test_util.py

@@ -0,0 +1,14 @@
+import numpy as np
+
+from pandas import DataFrame
+from pandas.tests.copy_view.util import get_array
+
+
+def test_get_array_numpy():
+    df = DataFrame({"a": [1, 2, 3]})
+    assert np.shares_memory(get_array(df, "a"), get_array(df, "a"))
+
+
+def test_get_array_masked():
+    df = DataFrame({"a": [1, 2, 3]}, dtype="Int64")
+    assert np.shares_memory(get_array(df, "a"), get_array(df, "a"))

venv/lib/python3.12/site-packages/pandas/tests/copy_view/util.py

@@ -0,0 +1,30 @@
+from pandas import (
+    Categorical,
+    Index,
+    Series,
+)
+from pandas.core.arrays import BaseMaskedArray
+
+
+def get_array(obj, col=None):
+    """
+    Helper method to get array for a DataFrame column or a Series.
+
+    Equivalent of df[col].values, but without going through normal getitem,
+    which triggers tracking references / CoW (and we might be testing that
+    this is done by some other operation).
+    """
+    if isinstance(obj, Index):
+        arr = obj._values
+    elif isinstance(obj, Series) and (col is None or obj.name == col):
+        arr = obj._values
+    else:
+        assert col is not None
+        icol = obj.columns.get_loc(col)
+        assert isinstance(icol, int)
+        arr = obj._get_column_array(icol)
+    if isinstance(arr, BaseMaskedArray):
+        return arr._data
+    elif isinstance(arr, Categorical):
+        return arr
+    return getattr(arr, "_ndarray", arr)