CLN: Misc copy-on-write cleanups (#58741)

* CLN: Remove using_cow

* Remove unused contexts

* Remove some COW comments in arrow

* Remove other copy

Author: mroeschke

pandas/_config/__init__.py

@@ -15,7 +15,6 @@
     "describe_option",
     "option_context",
     "options",
-    "using_copy_on_write",
 ]
 from pandas._config import config
 from pandas._config import dates  # pyright: ignore[reportUnusedImport]  # noqa: F401
@@ -31,10 +30,6 @@
 from pandas._config.display import detect_console_encoding
 
 
-def using_copy_on_write() -> bool:
-    return True
-
-
 def using_pyarrow_string_dtype() -> bool:
     _mode_options = _global_config["future"]
     return _mode_options["infer_string"]

pandas/_testing/contexts.py

@@ -11,8 +11,6 @@
 )
 import uuid
 
-from pandas._config import using_copy_on_write
-
 from pandas.compat import PYPY
 from pandas.errors import ChainedAssignmentError
 
@@ -158,34 +156,25 @@ def with_csv_dialect(name: str, **kwargs) -> Generator[None, None, None]:
         csv.unregister_dialect(name)
 
 
-def raises_chained_assignment_error(warn=True, extra_warnings=(), extra_match=()):
+def raises_chained_assignment_error(extra_warnings=(), extra_match=()):
     from pandas._testing import assert_produces_warning
 
-    if not warn:
-        from contextlib import nullcontext
-
-        return nullcontext()
-
-    if PYPY and not extra_warnings:
-        from contextlib import nullcontext
+    if PYPY:
+        if not extra_warnings:
+            from contextlib import nullcontext
 
-        return nullcontext()
-    elif PYPY and extra_warnings:
-        return assert_produces_warning(
-            extra_warnings,
-            match=extra_match,
-        )
-    else:
-        if using_copy_on_write():
-            warning = ChainedAssignmentError
-            match = (
-                "A value is trying to be set on a copy of a DataFrame or Series "
-                "through chained assignment"
-            )
+            return nullcontext()
         else:
-            warning = FutureWarning  # type: ignore[assignment]
-            # TODO update match
-            match = "ChainedAssignmentError"
+            return assert_produces_warning(
+                extra_warnings,
+                match=extra_match,
+            )
+    else:
+        warning = ChainedAssignmentError
+        match = (
+            "A value is trying to be set on a copy of a DataFrame or Series "
+            "through chained assignment"
+        )
         if extra_warnings:
             warning = (warning, *extra_warnings)  # type: ignore[assignment]
         return assert_produces_warning(

pandas/core/arrays/arrow/array.py

@@ -1052,8 +1052,7 @@ def _pad_or_backfill(
         copy: bool = True,
     ) -> Self:
         if not self._hasna:
-            # TODO(CoW): Not necessary anymore when CoW is the default
-            return self.copy()
+            return self
 
         if limit is None and limit_area is None:
             method = missing.clean_fill_method(method)
@@ -1084,7 +1083,6 @@ def fillna(
         copy: bool = True,
     ) -> Self:
         if not self._hasna:
-            # TODO(CoW): Not necessary anymore when CoW is the default
             return self.copy()
 
         if limit is not None:

pandas/core/indexing.py

@@ -2372,8 +2372,7 @@ def ravel(i):
             # we have a frame, with multiple indexers on both axes; and a
             # series, so need to broadcast (see GH5206)
             if sum_aligners == self.ndim and all(is_sequence(_) for _ in indexer):
-                # TODO(CoW): copy shouldn't be needed here
-                ser_values = ser.reindex(obj.axes[0][indexer[0]]).copy()._values
+                ser_values = ser.reindex(obj.axes[0][indexer[0]])._values
 
                 # single indexer
                 if len(indexer) > 1 and not multiindex_indexer:

pandas/tests/copy_view/test_astype.py

@@ -6,7 +6,6 @@
 from pandas.compat.pyarrow import pa_version_under12p0
 import pandas.util._test_decorators as td
 
-import pandas as pd
 from pandas import (
     DataFrame,
     Series,
@@ -79,8 +78,7 @@ def test_astype_different_target_dtype(dtype):
 
 def test_astype_numpy_to_ea():
     ser = Series([1, 2, 3])
-    with pd.option_context("mode.copy_on_write", True):
-        result = ser.astype("Int64")
+    result = ser.astype("Int64")
     assert np.shares_memory(get_array(ser), get_array(result))
 
 

pandas/tests/indexes/base_class/test_constructors.py

@@ -75,6 +75,5 @@ def test_inference_on_pandas_objects(self):
     def test_constructor_not_read_only(self):
         # GH#57130
         ser = Series([1, 2], dtype=object)
-        with pd.option_context("mode.copy_on_write", True):
-            idx = Index(ser)
-            assert idx._values.flags.writeable
+        idx = Index(ser)
+        assert idx._values.flags.writeable

pandas/tests/indexing/test_chaining_and_caching.py

@@ -284,6 +284,7 @@ def test_detect_chained_assignment_changing_dtype(self):
         with tm.raises_chained_assignment_error():
             df.loc[2]["C"] = "foo"
         tm.assert_frame_equal(df, df_original)
+        # TODO: Use tm.raises_chained_assignment_error() when PDEP-6 is enforced
         with tm.raises_chained_assignment_error(
             extra_warnings=(FutureWarning,), extra_match=(None,)
         ):