From 0c8e753a632704c5538670836f3f725b57159ccf Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Wed, 27 Dec 2023 16:47:25 +0100
Subject: [PATCH 1/8] CoW: Enable CoW by default and remove warning build

---
 .github/workflows/unit-tests.yml         | 12 ----------
 pandas/_config/__init__.py               | 12 ++--------
 pandas/conftest.py                       | 12 ++--------
 pandas/tests/copy_view/test_internals.py | 30 ------------------------
 pandas/tests/extension/conftest.py       | 12 ++--------
 pandas/tests/test_downstream.py          |  1 +
 6 files changed, 7 insertions(+), 72 deletions(-)

diff --git a/.github/workflows/unit-tests.yml b/.github/workflows/unit-tests.yml
index 6ca4d19196874..ec02d75926ad3 100644
--- a/.github/workflows/unit-tests.yml
+++ b/.github/workflows/unit-tests.yml
@@ -73,18 +73,6 @@ jobs:
             env_file: actions-312.yaml
             pattern: "not slow and not network and not single_cpu"
             pandas_copy_on_write: "1"
-          - name: "Copy-on-Write 3.11 (warnings)"
-            env_file: actions-311.yaml
-            pattern: "not slow and not network and not single_cpu"
-            pandas_copy_on_write: "warn"
-          - name: "Copy-on-Write 3.10 (warnings)"
-            env_file: actions-310.yaml
-            pattern: "not slow and not network and not single_cpu"
-            pandas_copy_on_write: "warn"
-          - name: "Copy-on-Write 3.9 (warnings)"
-            env_file: actions-39.yaml
-            pattern: "not slow and not network and not single_cpu"
-            pandas_copy_on_write: "warn"
           - name: "Pypy"
             env_file: actions-pypy-39.yaml
             pattern: "not slow and not network and not single_cpu"
diff --git a/pandas/_config/__init__.py b/pandas/_config/__init__.py
index 97784c924dab4..0594d1c190a72 100644
--- a/pandas/_config/__init__.py
+++ b/pandas/_config/__init__.py
@@ -32,19 +32,11 @@
 
 
 def using_copy_on_write() -> bool:
-    _mode_options = _global_config["mode"]
-    return (
-        _mode_options["copy_on_write"] is True
-        and _mode_options["data_manager"] == "block"
-    )
+    return True
 
 
 def warn_copy_on_write() -> bool:
-    _mode_options = _global_config["mode"]
-    return (
-        _mode_options["copy_on_write"] == "warn"
-        and _mode_options["data_manager"] == "block"
-    )
+    return False
 
 
 def using_nullable_dtypes() -> bool:
diff --git a/pandas/conftest.py b/pandas/conftest.py
index 046cda259eefd..8d2520afb1b0a 100644
--- a/pandas/conftest.py
+++ b/pandas/conftest.py
@@ -48,8 +48,6 @@
     utc,
 )
 
-from pandas._config.config import _get_option
-
 import pandas.util._test_decorators as td
 
 from pandas.core.dtypes.dtypes import (
@@ -1882,10 +1880,7 @@ def using_copy_on_write() -> bool:
     """
     Fixture to check if Copy-on-Write is enabled.
     """
-    return (
-        pd.options.mode.copy_on_write is True
-        and _get_option("mode.data_manager", silent=True) == "block"
-    )
+    return True
 
 
 @pytest.fixture
@@ -1893,10 +1888,7 @@ def warn_copy_on_write() -> bool:
     """
     Fixture to check if Copy-on-Write is in warning mode.
     """
-    return (
-        pd.options.mode.copy_on_write == "warn"
-        and _get_option("mode.data_manager", silent=True) == "block"
-    )
+    return False
 
 
 @pytest.fixture
diff --git a/pandas/tests/copy_view/test_internals.py b/pandas/tests/copy_view/test_internals.py
index 615b024bd06bf..324d5ac7f5d3c 100644
--- a/pandas/tests/copy_view/test_internals.py
+++ b/pandas/tests/copy_view/test_internals.py
@@ -1,7 +1,6 @@
 import numpy as np
 import pytest
 
-import pandas as pd
 from pandas import DataFrame
 import pandas._testing as tm
 from pandas.tests.copy_view.util import get_array
@@ -42,35 +41,6 @@ def test_consolidate(using_copy_on_write):
         assert df.loc[0, "b"] == 0.1
 
 
-@pytest.mark.single_cpu
-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
-
-
 @pytest.mark.parametrize("dtype", [np.intp, np.int8])
 @pytest.mark.parametrize(
     "locs, arr",
diff --git a/pandas/tests/extension/conftest.py b/pandas/tests/extension/conftest.py
index c5b1295ee4a7d..3a3844d5a8b7a 100644
--- a/pandas/tests/extension/conftest.py
+++ b/pandas/tests/extension/conftest.py
@@ -2,12 +2,7 @@
 
 import pytest
 
-from pandas._config.config import _get_option
-
-from pandas import (
-    Series,
-    options,
-)
+from pandas import Series
 
 
 @pytest.fixture
@@ -224,7 +219,4 @@ def using_copy_on_write() -> bool:
     """
     Fixture to check if Copy-on-Write is enabled.
     """
-    return (
-        options.mode.copy_on_write is True
-        and _get_option("mode.data_manager", silent=True) == "block"
-    )
+    return True
diff --git a/pandas/tests/test_downstream.py b/pandas/tests/test_downstream.py
index 51ce73ef54300..1c7ff3f924dc3 100644
--- a/pandas/tests/test_downstream.py
+++ b/pandas/tests/test_downstream.py
@@ -46,6 +46,7 @@ def test_dask(df):
         pd.set_option("compute.use_numexpr", olduse)
 
 
+@pytest.mark.xfail(reason="array is read-only")
 def test_dask_ufunc():
     # dask sets "compute.use_numexpr" to False, so catch the current value
     # and ensure to reset it afterwards to avoid impacting other tests

From 7929800bc03f7a5c3a456fbc953121799951b6ed Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Wed, 27 Dec 2023 17:13:58 +0100
Subject: [PATCH 2/8] CoW: Boolean indexer in MultiIndex raising read-only
 error

---
 pandas/core/indexes/multi.py            |  2 ++
 pandas/tests/copy_view/test_indexing.py | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 2a4e027e2b806..681a3287e46e6 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -3487,6 +3487,8 @@ def _to_bool_indexer(indexer) -> npt.NDArray[np.bool_]:
                         "cannot index with a boolean indexer that "
                         "is not the same length as the index"
                     )
+                if isinstance(k, (ABCSeries, Index)):
+                    k = k._values
                 lvl_indexer = np.asarray(k)
 
             elif is_list_like(k):
diff --git a/pandas/tests/copy_view/test_indexing.py b/pandas/tests/copy_view/test_indexing.py
index 9afc98e558c11..7f7be355b76c7 100644
--- a/pandas/tests/copy_view/test_indexing.py
+++ b/pandas/tests/copy_view/test_indexing.py
@@ -1180,6 +1180,24 @@ def test_series_midx_tuples_slice(using_copy_on_write, warn_copy_on_write):
         tm.assert_series_equal(ser, expected)
 
 
+def test_midx_read_only_bool_indexer():
+    def mklbl(prefix, n):
+        return [f"{prefix}{i}" for i in range(n)]
+
+    idx = pd.MultiIndex.from_product(
+        [mklbl("A", 4), mklbl("B", 2), mklbl("C", 4), mklbl("D", 2)]
+    )
+    cols = pd.MultiIndex.from_tuples(
+        [("a", "foo"), ("a", "bar"), ("b", "foo"), ("b", "bah")], names=["lvl0", "lvl1"]
+    )
+    df = DataFrame(1, index=idx, columns=cols).sort_index().sort_index(axis=1)
+
+    mask = df[("a", "foo")] == 1
+    result = df.loc[pd.IndexSlice[mask, :, ["C1", "C3"]], :]
+    expected = df.loc[pd.IndexSlice[:, :, ["C1", "C3"]], :]
+    tm.assert_frame_equal(result, expected)
+
+
 def test_loc_enlarging_with_dataframe(using_copy_on_write):
     df = DataFrame({"a": [1, 2, 3]})
     rhs = DataFrame({"b": [1, 2, 3], "c": [4, 5, 6]})

From d9e0570812358dba91a8b771f7c0a70dc06a8f52 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Wed, 27 Dec 2023 17:27:14 +0100
Subject: [PATCH 3/8] Fixup ci

---
 pandas/tests/frame/methods/test_copy.py | 22 ----------------------
 pandas/tests/series/test_ufunc.py       |  5 +----
 pandas/util/_test_decorators.py         | 14 --------------
 3 files changed, 1 insertion(+), 40 deletions(-)

diff --git a/pandas/tests/frame/methods/test_copy.py b/pandas/tests/frame/methods/test_copy.py
index 6208d0256a655..5b72a84320c52 100644
--- a/pandas/tests/frame/methods/test_copy.py
+++ b/pandas/tests/frame/methods/test_copy.py
@@ -1,10 +1,7 @@
 import numpy as np
 import pytest
 
-import pandas.util._test_decorators as td
-
 from pandas import DataFrame
-import pandas._testing as tm
 
 
 class TestCopy:
@@ -18,25 +15,6 @@ def test_copy_index_name_checking(self, float_frame, attr):
         getattr(cp, attr).name = "foo"
         assert getattr(float_frame, attr).name is None
 
-    @td.skip_copy_on_write_invalid_test
-    def test_copy_cache(self):
-        # GH#31784 _item_cache not cleared on copy causes incorrect reads after updates
-        df = DataFrame({"a": [1]})
-
-        df["x"] = [0]
-        df["a"]
-
-        df.copy()
-
-        df["a"].values[0] = -1
-
-        tm.assert_frame_equal(df, DataFrame({"a": [-1], "x": [0]}))
-
-        df["y"] = [0]
-
-        assert df["a"].values[0] == -1
-        tm.assert_frame_equal(df, DataFrame({"a": [-1], "x": [0], "y": [0]}))
-
     def test_copy(self, float_frame, float_string_frame):
         cop = float_frame.copy()
         cop["E"] = cop["A"]
diff --git a/pandas/tests/series/test_ufunc.py b/pandas/tests/series/test_ufunc.py
index 9d13ebf740eab..ede6b98b7bf8e 100644
--- a/pandas/tests/series/test_ufunc.py
+++ b/pandas/tests/series/test_ufunc.py
@@ -5,8 +5,6 @@
 import numpy as np
 import pytest
 
-import pandas.util._test_decorators as td
-
 import pandas as pd
 import pandas._testing as tm
 from pandas.arrays import SparseArray
@@ -449,8 +447,7 @@ def add3(x, y, z):
         ufunc(ser, ser, df)
 
 
-# TODO(CoW) see https://github.com/pandas-dev/pandas/pull/51082
-@td.skip_copy_on_write_not_yet_implemented
+@pytest.mark.xfail(reason="see https://github.com/pandas-dev/pandas/pull/51082")
 def test_np_fix():
     # np.fix is not a ufunc but is composed of several ufunc calls under the hood
     # with `out` and `where` keywords
diff --git a/pandas/util/_test_decorators.py b/pandas/util/_test_decorators.py
index 37908c9ac255b..78626781289c4 100644
--- a/pandas/util/_test_decorators.py
+++ b/pandas/util/_test_decorators.py
@@ -33,12 +33,9 @@ def test_foo():
 
 import pytest
 
-from pandas._config import get_option
-
 if TYPE_CHECKING:
     from pandas._typing import F
 
-
 from pandas.compat import (
     IS64,
     is_platform_windows,
@@ -144,14 +141,3 @@ def documented_fixture(fixture):
         return fixture
 
     return documented_fixture
-
-
-skip_copy_on_write_not_yet_implemented = pytest.mark.xfail(
-    get_option("mode.copy_on_write") is True,
-    reason="Not yet implemented/adapted for Copy-on-Write mode",
-)
-
-skip_copy_on_write_invalid_test = pytest.mark.skipif(
-    get_option("mode.copy_on_write") is True,
-    reason="Test not valid for Copy-on-Write mode",
-)

From 1384b66c97724790f68cf67e40743cc0b5abf7f6 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Wed, 27 Dec 2023 20:30:29 +0100
Subject: [PATCH 4/8] Update docs

---
 pandas/core/generic.py | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index de25a02c6b37c..2fe0b5cd5443a 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -6710,8 +6710,7 @@ def copy(self, deep: bool_t | None = True) -> Self:
         :ref:`gotchas <gotchas.thread-safety>` when copying in a threading
         environment.
 
-        When ``copy_on_write`` in pandas config is set to ``True``, the
-        ``copy_on_write`` config takes effect even when ``deep=False``.
+        Copy-on-Write protects shallow copies against accidental modifications.
         This means that any changes to the copied data would make a new copy
         of the data upon write (and vice versa). Changes made to either the
         original or copied variable would not be reflected in the counterpart.
@@ -6737,11 +6736,14 @@ def copy(self, deep: bool_t | None = True) -> Self:
         >>> deep = s.copy()
         >>> shallow = s.copy(deep=False)
 
-        Shallow copy shares data and index with original.
+        Shallow copy shares index with original, the data is a
+        view of the original.
 
         >>> s is shallow
         False
-        >>> s.values is shallow.values and s.index is shallow.index
+        >>> s.values is shallow.values
+        False
+        >>>  s.index is shallow.index
         True
 
         Deep copy has own copy of data and index.
@@ -6751,18 +6753,17 @@ def copy(self, deep: bool_t | None = True) -> Self:
         >>> s.values is deep.values or s.index is deep.index
         False
 
-        Updates to the data shared by shallow copy and original is reflected
-        in both (NOTE: this will no longer be true for pandas >= 3.0);
-        deep copy remains unchanged.
+        The shallow copy is protected against updating the original object
+        as well. Thus, updates will only reflect in one of both objects.
 
         >>> s.iloc[0] = 3
         >>> shallow.iloc[1] = 4
         >>> s
         a    3
-        b    4
+        b    2
         dtype: int64
         >>> shallow
-        a    3
+        a    1
         b    4
         dtype: int64
         >>> deep

From 6936c873ed062d30fa7fe943d9261427acb930b8 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Wed, 27 Dec 2023 20:40:40 +0100
Subject: [PATCH 5/8] Fixup asvs

---
 asv_bench/benchmarks/algos/isin.py | 3 ++-
 asv_bench/benchmarks/strings.py    | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/asv_bench/benchmarks/algos/isin.py b/asv_bench/benchmarks/algos/isin.py
index 2b3d32fb579dc..d650732e9c875 100644
--- a/asv_bench/benchmarks/algos/isin.py
+++ b/asv_bench/benchmarks/algos/isin.py
@@ -59,7 +59,8 @@ def setup(self, dtype):
         elif dtype in ["str", "string[python]", "string[pyarrow]"]:
             try:
                 self.series = Series(
-                    Index([f"i-{i}" for i in range(N)], dtype=object), dtype=dtype
+                    Index([f"i-{i}" for i in range(N)], dtype=object)._values,
+                    dtype=dtype,
                 )
             except ImportError:
                 raise NotImplementedError
diff --git a/asv_bench/benchmarks/strings.py b/asv_bench/benchmarks/strings.py
index 1f4a104255057..accc970f71d60 100644
--- a/asv_bench/benchmarks/strings.py
+++ b/asv_bench/benchmarks/strings.py
@@ -19,7 +19,8 @@ class Dtypes:
     def setup(self, dtype):
         try:
             self.s = Series(
-                Index([f"i-{i}" for i in range(10000)], dtype=object), dtype=dtype
+                Index([f"i-{i}" for i in range(10000)], dtype=object)._values,
+                dtype=dtype,
             )
         except ImportError:
             raise NotImplementedError

From f68c2fece71e2175e91e0b9eeb0e10fdede4bb37 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Tue, 30 Jan 2024 00:13:05 +0000
Subject: [PATCH 6/8] Update docstring

---
 pandas/core/generic.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 2fe0b5cd5443a..c517b56a2b91a 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -6743,7 +6743,7 @@ def copy(self, deep: bool_t | None = True) -> Self:
         False
         >>> s.values is shallow.values
         False
-        >>>  s.index is shallow.index
+        >>> s.index is shallow.index
         True
 
         Deep copy has own copy of data and index.

From 8c6fd9bc33ee09958459c4d1cca0ce735c8f5867 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Tue, 30 Jan 2024 20:57:36 +0000
Subject: [PATCH 7/8] Update docs

---
 pandas/core/generic.py | 18 +-----------------
 1 file changed, 1 insertion(+), 17 deletions(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index d9ef33d9bc21e..31ed17268dc63 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -6769,7 +6769,7 @@ def copy(self, deep: bool_t | None = True) -> Self:
         >>> s.values is shallow.values
         False
         >>> s.index is shallow.index
-        True
+        False
 
         Deep copy has own copy of data and index.
 
@@ -6811,22 +6811,6 @@ def copy(self, deep: bool_t | None = True) -> Self:
         0    [10, 2]
         1     [3, 4]
         dtype: object
-
-        **Copy-on-Write is set to true**, the shallow copy is not modified
-        when the original data is changed:
-
-        >>> with pd.option_context("mode.copy_on_write", True):
-        ...     s = pd.Series([1, 2], index=["a", "b"])
-        ...     copy = s.copy(deep=False)
-        ...     s.iloc[0] = 100
-        ...     s
-        a    100
-        b      2
-        dtype: int64
-        >>> copy
-        a    1
-        b    2
-        dtype: int64
         """
         data = self._mgr.copy(deep=deep)
         self._clear_item_cache()

From ba224b88a59af6cd7207b2e6eae2bc5c296af714 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Wed, 31 Jan 2024 22:13:55 +0000
Subject: [PATCH 8/8] Use other function

---
 pandas/tests/test_downstream.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/test_downstream.py b/pandas/tests/test_downstream.py
index c904784d44dea..feba0e86c6b32 100644
--- a/pandas/tests/test_downstream.py
+++ b/pandas/tests/test_downstream.py
@@ -46,7 +46,6 @@ def test_dask(df):
         pd.set_option("compute.use_numexpr", olduse)
 
 
-@pytest.mark.xfail(reason="array is read-only")
 def test_dask_ufunc():
     # dask sets "compute.use_numexpr" to False, so catch the current value
     # and ensure to reset it afterwards to avoid impacting other tests
@@ -59,8 +58,8 @@ def test_dask_ufunc():
         s = Series([1.5, 2.3, 3.7, 4.0])
         ds = dd.from_pandas(s, npartitions=2)
 
-        result = da.fix(ds).compute()
-        expected = np.fix(s)
+        result = da.log(ds).compute()
+        expected = np.log(s)
         tm.assert_series_equal(result, expected)
     finally:
         pd.set_option("compute.use_numexpr", olduse)