CoW: add warning mode for cases that will change behaviour

pandas/_config/__init__.py

@@ -15,6 +15,7 @@
     "option_context",
     "options",
     "using_copy_on_write",
+    "warn_copy_on_write",
 ]
 from pandas._config import config
 from pandas._config import dates  # pyright: ignore[reportUnusedImport]  # noqa: F401
@@ -32,7 +33,18 @@
 
 def using_copy_on_write() -> bool:
     _mode_options = _global_config["mode"]
-    return _mode_options["copy_on_write"] and _mode_options["data_manager"] == "block"
+    return (
+        _mode_options["copy_on_write"] is True
+        and _mode_options["data_manager"] == "block"
+    )
+
+
+def warn_copy_on_write() -> bool:
+    _mode_options = _global_config["mode"]
+    return (
+        _mode_options["copy_on_write"] == "warn"
+        and _mode_options["data_manager"] == "block"
+    )
 
 
 def using_nullable_dtypes() -> bool:

pandas/conftest.py

@@ -1994,7 +1994,18 @@ def using_copy_on_write() -> bool:
     Fixture to check if Copy-on-Write is enabled.
     """
     return (
-        pd.options.mode.copy_on_write
+        pd.options.mode.copy_on_write is True
+        and _get_option("mode.data_manager", silent=True) == "block"
+    )
+
+
+@pytest.fixture
+def warn_copy_on_write() -> bool:
+    """
+    Fixture to check if Copy-on-Write is enabled.
+    """
+    return (
+        pd.options.mode.copy_on_write == "warn"
         and _get_option("mode.data_manager", silent=True) == "block"
     )
 

pandas/core/config_init.py

@@ -476,9 +476,11 @@ def use_inf_as_na_cb(key) -> None:
         "copy_on_write",
         # Get the default from an environment variable, if set, otherwise defaults
         # to False. This environment variable can be set for testing.
-        os.environ.get("PANDAS_COPY_ON_WRITE", "0") == "1",
+        "warn"
+        if os.environ.get("PANDAS_COPY_ON_WRITE", "0") == "warn"
+        else os.environ.get("PANDAS_COPY_ON_WRITE", "0") == "1",
         copy_on_write_doc,
-        validator=is_bool,
+        validator=is_one_of_factory([True, False, "warn"]),
     )
 
 

pandas/core/generic.py

@@ -30,6 +30,7 @@
 from pandas._config import (
     config,
     using_copy_on_write,
+    warn_copy_on_write,
 )
 
 from pandas._libs import lib
@@ -4396,7 +4397,7 @@ def _check_setitem_copy(self, t: str = "setting", force: bool_t = False):
         df.iloc[0:5]['group'] = 'a'
 
         """
-        if using_copy_on_write():
+        if using_copy_on_write() or warn_copy_on_write():
             return
 
         # return early if the check is not needed

pandas/core/internals/managers.py

@@ -16,7 +16,10 @@
 
 import numpy as np
 
-from pandas._config import using_copy_on_write
+from pandas._config import (
+    using_copy_on_write,
+    warn_copy_on_write,
+)
 
 from pandas._libs import (
     internals as libinternals,
@@ -1988,9 +1991,17 @@ def setitem_inplace(self, indexer, value) -> None:
         in place, not returning a new Manager (and Block), and thus never changing
         the dtype.
         """
-        if using_copy_on_write() and not self._has_no_reference(0):
-            self.blocks = (self._block.copy(),)
-            self._cache.clear()
+        if not self._has_no_reference(0):
+            if using_copy_on_write():
+                self.blocks = (self._block.copy(),)
+                self._cache.clear()
+            elif warn_copy_on_write():
+                warnings.warn(
+                    "Setting value on view: behaviour will change in pandas 3.0 "
+                    "with Copy-on-Write ...",
+                    FutureWarning,
+                    stacklevel=find_stack_level(),
+                )
 
         super().setitem_inplace(indexer, value)
 

pandas/tests/copy_view/test_indexing.py

@@ -879,7 +879,9 @@ def test_del_series(backend):
 # Accessing column as Series
 
 
-def test_column_as_series(backend, using_copy_on_write, using_array_manager):
+def test_column_as_series(
+    backend, using_copy_on_write, warn_copy_on_write, using_array_manager
+):
     # Case: selecting a single column now also uses Copy-on-Write
     dtype_backend, DataFrame, Series = backend
     df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [0.1, 0.2, 0.3]})
@@ -892,10 +894,14 @@ def test_column_as_series(backend, using_copy_on_write, using_array_manager):
     if using_copy_on_write or using_array_manager:
         s[0] = 0
     else:
-        warn = SettingWithCopyWarning if dtype_backend == "numpy" else None
-        with pd.option_context("chained_assignment", "warn"):
-            with tm.assert_produces_warning(warn):
+        if warn_copy_on_write:
+            with tm.assert_produces_warning(FutureWarning):
                 s[0] = 0
+        else:
+            warn = SettingWithCopyWarning if dtype_backend == "numpy" else None
+            with pd.option_context("chained_assignment", "warn"):
+                with tm.assert_produces_warning(warn):
+                    s[0] = 0
 
     expected = Series([0, 2, 3], name="a")
     tm.assert_series_equal(s, expected)

pandas/tests/extension/conftest.py

@@ -215,6 +215,6 @@ def using_copy_on_write() -> bool:
     Fixture to check if Copy-on-Write is enabled.
     """
     return (
-        options.mode.copy_on_write
+        options.mode.copy_on_write is True
         and _get_option("mode.data_manager", silent=True) == "block"
     )