DEPR: unused Copy-on-Write option (#63231)

Author: rhshadrach

ci/code_checks.sh

@@ -72,6 +72,7 @@ if [[ -z "$CHECK" || "$CHECK" == "docstrings" ]]; then
         -i "pandas.Series.dt PR01" `# Accessors are implemented as classes, but we do not document the Parameters section` \
         -i "pandas.Period.freq GL08" \
         -i "pandas.Period.ordinal GL08" \
+        -i "pandas.errors.ChainedAssignmentError SA01" \
         -i "pandas.errors.IncompatibleFrequency SA01,SS06,EX01" \
         -i "pandas.api.extensions.ExtensionArray.value_counts EX01,RT03,SA01" \
         -i "pandas.api.typing.DataFrameGroupBy.plot PR02" \

doc/source/whatsnew/v3.0.0.rst

@@ -117,6 +117,9 @@ process in more detail.
 
     `PDEP-7: Consistent copy/view semantics in pandas with Copy-on-Write <https://pandas.pydata.org/pdeps/0007-copy-on-write.html>`__
 
+Setting the option ``mode.copy_on_write`` no longer has any impact. The option is deprecated
+and will be removed in pandas 4.0.
+
 .. _whatsnew_300.enhancements.col:
 
 ``pd.col`` syntax can now be used in :meth:`DataFrame.assign` and :meth:`DataFrame.loc`

pandas/core/config_init.py

@@ -408,12 +408,11 @@ def is_terminal() -> bool:
     cf.register_option("sim_interactive", False, tc_sim_interactive_doc)
 
 
-# TODO better name?
 copy_on_write_doc = """
 : bool
-    Use new copy-view behaviour using Copy-on-Write. Defaults to False,
-    unless overridden by the 'PANDAS_COPY_ON_WRITE' environment variable
-    (if set to "1" for True, needs to be set before pandas is imported).
+    Use new copy-view behaviour using Copy-on-Write. No longer used,
+    pandas now always uses Copy-on-Write behavior. This option will
+    be removed in pandas 4.0.
 """
 
 
@@ -902,3 +901,11 @@ def register_converter_cb(key: str) -> None:
 
 # GH#59502
 cf.deprecate_option("future.no_silent_downcasting", Pandas4Warning)
+cf.deprecate_option(
+    "mode.copy_on_write",
+    Pandas4Warning,
+    msg=(
+        "Copy-on-Write can no longer be disabled, setting to False has no impact. "
+        "This option will be removed in pandas 4.0."
+    ),
+)

pandas/errors/__init__.py

@@ -668,7 +668,7 @@ class ChainedAssignmentError(Warning):
     """
     Warning raised when trying to set using chained assignment.
 
-    When the ``mode.copy_on_write`` option is enabled, chained assignment can
+    With Copy-on-Write now always enabled, chained assignment can
     never work. In such a situation, we are always setting into a temporary
     object that is the result of an indexing operation (getitem), which under
     Copy-on-Write always behaves as a copy. Thus, assigning through a chain
@@ -677,18 +677,11 @@ class ChainedAssignmentError(Warning):
     For more information on Copy-on-Write,
     see :ref:`the user guide<copy_on_write>`.
 
-    See Also
-    --------
-    options.mode.copy_on_write : Global setting for enabling or disabling
-        Copy-on-Write behavior.
-
     Examples
     --------
-    >>> pd.options.mode.copy_on_write = True
     >>> df = pd.DataFrame({"A": [1, 1, 1, 2, 2]}, columns=["A"])
     >>> df["A"][0:3] = 10  # doctest: +SKIP
     ... # ChainedAssignmentError: ...
-    >>> pd.options.mode.copy_on_write = False
     """
 
 

pandas/tests/copy_view/test_copy_deprecation.py

@@ -89,3 +89,12 @@ def test_copy_deprecation_merge_concat():
         Pandas4Warning, match="copy", check_stacklevel=False
     ):
         concat([df, df], copy=False)
+
+
+@pytest.mark.parametrize("value", [False, True, "warn"])
+def test_copy_on_write_deprecation_option(value):
+    msg = "Copy-on-Write can no longer be disabled"
+    # stacklevel points to contextlib due to use of context manager.
+    with tm.assert_produces_warning(Pandas4Warning, match=msg, check_stacklevel=False):
+        with pd.option_context("mode.copy_on_write", value):
+            pass