DEP: Deprecate CoW and no_silent_downcasting options

doc/source/whatsnew/v1.5.0.rst

@@ -346,13 +346,19 @@ Without copy on write, the parent :class:`DataFrame` is updated when updating a
 
 With copy on write enabled, df won't be updated anymore:
 
-.. ipython:: python
+.. code-block:: python
 
-    with pd.option_context("mode.copy_on_write", True):
-        df = pd.DataFrame({"foo": [1, 2, 3], "bar": 1})
-        view = df["foo"]
-        view.iloc[0]
-        df
+    In [2]: with pd.option_context("mode.copy_on_write", True):
+       ...:    df = pd.DataFrame({"foo": [1, 2, 3], "bar": 1})
+       ...:    view = df["foo"]
+       ...:    view.iloc[0]
+       ...:    df
+
+    Out[3]:
+       foo  bar
+    0    1    1
+    1    2    1
+    2    3    1
 
 A more detailed explanation can be found `here <https://phofl.github.io/cow-introduction.html>`_.
 

doc/source/whatsnew/v3.0.0.rst

@@ -185,7 +185,8 @@ Other Deprecations
 - Deprecated :meth:`Timestamp.utcnow`, use ``Timestamp.now("UTC")`` instead (:issue:`56680`)
 - Deprecated allowing non-keyword arguments in :meth:`Series.to_markdown` except ``buf``. (:issue:`57280`)
 - Deprecated allowing non-keyword arguments in :meth:`Series.to_string` except ``buf``. (:issue:`57280`)
--
+- Deprecated the option ``future.no_silent_downcasting``. The opt-in behavior from the 2.x release series is already the default and this option has no effect anymore (:issue:`57872`)
+- Deprecated the option ``mode.copy_on_write``. Copy-on-Write is already the default and this option has no effect anymore (:issue:`57872`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.prior_deprecations:

pandas/__init__.py

@@ -2,6 +2,9 @@
 
 __docformat__ = "restructuredtext"
 
+import os
+import warnings
+
 # Let users know if they're missing any of our hard dependencies
 _hard_dependencies = ("numpy", "pytz", "dateutil")
 _missing_dependencies = []
@@ -190,6 +193,18 @@
     del get_versions, v
 
 
+if "PANDAS_COPY_ON_WRITE" in os.environ:
+    warnings.warn(
+        "The env variable PANDAS_COPY_ON_WRITE is set. The copy_on_write option is "
+        "deprecated and will be removed in a future version. Copy-on-Write "
+        "is already enabled by default.",
+        FutureWarning,
+        stacklevel=2,
+    )
+
+del os, warnings
+
+
 # module level doc-string
 __doc__ = """
 pandas - a powerful data analysis and manipulation library for Python

pandas/core/config_init.py

@@ -875,3 +875,21 @@ def register_converter_cb(key: str) -> None:
         "(at which point this option will be deprecated).",
         validator=is_one_of_factory([True, False]),
     )
+
+
+cf.deprecate_option(
+    # GH#55043
+    "mode.copy_on_write",
+    "copy_on_write option is deprecated and will be removed in a future "
+    "version. Copy-on-Write is already enabled; this option has no effect "
+    "anymore.",
+)
+
+
+cf.deprecate_option(
+    # GH#55043
+    "future.no_silent_downcasting",
+    "no_silent_downcasting option is deprecated and will be removed in a future "
+    "version. The deprecation is enforced and this option doesn't have any effect "
+    "anymore.",
+)

pandas/errors/__init__.py

@@ -434,11 +434,9 @@ class ChainedAssignmentError(Warning):
 
     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_option_deprecation.py

@@ -0,0 +1,24 @@
+import importlib
+
+import pytest
+
+import pandas as pd
+import pandas._testing as tm
+
+
+def test_configs_deprecated():
+    # GH#57872
+    with tm.assert_produces_warning(FutureWarning, match="is deprecated"):
+        pd.options.mode.copy_on_write = True
+
+    with tm.assert_produces_warning(FutureWarning, match="is deprecated"):
+        pd.options.future.no_silent_downcasting = True
+
+
+def test_env_variable():
+    # GH#57872
+    pytest.MonkeyPatch().setenv("PANDAS_COPY_ON_WRITE", "1")
+    with tm.assert_produces_warning(
+        FutureWarning, match="deprecated", check_stacklevel=False
+    ):
+        importlib.reload(pd)

pandas/tests/strings/test_api.py

@@ -8,7 +8,6 @@
     MultiIndex,
     Series,
     _testing as tm,
-    option_context,
 )
 from pandas.core.strings.accessor import StringMethods
 
@@ -164,8 +163,7 @@ def test_api_per_method(
 
     if inferred_dtype in allowed_types:
         # xref GH 23555, GH 23556
-        with option_context("future.no_silent_downcasting", True):
-            method(*args, **kwargs)  # works!
+        method(*args, **kwargs)  # works!
     else:
         # GH 23011, GH 23163
         msg = (