CoW: Enable CoW by default and remove warning build (#56633)

Author: phofl

.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"

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 as err:
                 raise NotImplementedError from err

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 as err:
             raise NotImplementedError from err

pandas/_config/__init__.py

@@ -32,13 +32,11 @@
 
 
 def using_copy_on_write() -> bool:
-    _mode_options = _global_config["mode"]
-    return _mode_options["copy_on_write"] is True
+    return True
 
 
 def warn_copy_on_write() -> bool:
-    _mode_options = _global_config["mode"]
-    return _mode_options["copy_on_write"] == "warn"
+    return False
 
 
 def using_nullable_dtypes() -> bool:

pandas/conftest.py

@@ -1963,15 +1963,15 @@ def using_copy_on_write() -> bool:
     """
     Fixture to check if Copy-on-Write is enabled.
     """
-    return pd.options.mode.copy_on_write is True
+    return True
 
 
 @pytest.fixture
 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"
+    return False
 
 
 @pytest.fixture

pandas/core/generic.py

@@ -6704,8 +6704,7 @@ 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.
@@ -6731,12 +6730,15 @@ 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
-        True
+        >>> s.values is shallow.values
+        False
+        >>> s.index is shallow.index
+        False
 
         Deep copy has own copy of data and index.
 
@@ -6745,18 +6747,17 @@ 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
@@ -6779,22 +6780,6 @@ 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()

pandas/core/indexes/multi.py

@@ -3492,6 +3492,8 @@ 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)
                 if indexer is None:
                     lvl_indexer = lvl_indexer.copy()

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",

pandas/tests/extension/conftest.py

@@ -2,10 +2,7 @@
 
 import pytest
 
-from pandas import (
-    Series,
-    options,
-)
+from pandas import Series
 
 
 @pytest.fixture
@@ -222,4 +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
+    return True

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"]