Remove Copy-on-Write warning mode (pandas-dev#57234)

pandas/_config/__init__.py

@@ -15,7 +15,6 @@
     "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
@@ -35,10 +34,6 @@ def using_copy_on_write() -> bool:
     return True
 
 
-def warn_copy_on_write() -> bool:
-    return False
-
-
 def using_nullable_dtypes() -> bool:
     _mode_options = _global_config["mode"]
     return _mode_options["nullable_dtypes"]

pandas/core/frame.py

@@ -42,7 +42,6 @@
 from pandas._config import (
     get_option,
     using_copy_on_write,
-    warn_copy_on_write,
 )
 
 from pandas._libs import (
@@ -64,7 +63,6 @@
     _chained_assignment_method_msg,
     _chained_assignment_msg,
     _chained_assignment_warning_method_msg,
-    _chained_assignment_warning_msg,
 )
 from pandas.util._decorators import (
     Appender,
@@ -4199,17 +4197,6 @@ def __setitem__(self, key, value) -> None:
                 warnings.warn(
                     _chained_assignment_msg, ChainedAssignmentError, stacklevel=2
                 )
-        elif not PYPY and not using_copy_on_write():
-            if sys.getrefcount(self) <= 3 and (
-                warn_copy_on_write()
-                or (
-                    not warn_copy_on_write()
-                    and any(b.refs.has_reference() for b in self._mgr.blocks)
-                )
-            ):
-                warnings.warn(
-                    _chained_assignment_warning_msg, FutureWarning, stacklevel=2
-                )
 
         key = com.apply_if_callable(key, self)
 
@@ -4550,7 +4537,7 @@ def _clear_item_cache(self) -> None:
 
     def _get_item_cache(self, item: Hashable) -> Series:
         """Return the cached item, item represents a label indexer."""
-        if using_copy_on_write() or warn_copy_on_write():
+        if using_copy_on_write():
             loc = self.columns.get_loc(item)
             return self._ixs(loc, axis=1)
 

pandas/core/generic.py

@@ -30,7 +30,6 @@
 from pandas._config import (
     config,
     using_copy_on_write,
-    warn_copy_on_write,
 )
 
 from pandas._libs import lib
@@ -105,7 +104,6 @@
 from pandas.errors.cow import (
     _chained_assignment_method_msg,
     _chained_assignment_warning_method_msg,
-    _check_cacher,
 )
 from pandas.util._decorators import (
     deprecate_nonkeyword_arguments,
@@ -4407,7 +4405,7 @@ def _check_setitem_copy(self, t: str = "setting", force: bool_t = False) -> None
         df.iloc[0:5]['group'] = 'a'
 
         """
-        if using_copy_on_write() or warn_copy_on_write():
+        if using_copy_on_write():
             return
 
         # return early if the check is not needed
@@ -7256,22 +7254,6 @@ def fillna(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
-            elif (
-                not PYPY
-                and not using_copy_on_write()
-                and self._is_view_after_cow_rules()
-            ):
-                ctr = sys.getrefcount(self)
-                ref_count = REF_COUNT
-                if isinstance(self, ABCSeries) and _check_cacher(self):
-                    # see https://github.com/pandas-dev/pandas/pull/56060#discussion_r1399245221
-                    ref_count += 1
-                if ctr <= ref_count:
-                    warnings.warn(
-                        _chained_assignment_warning_method_msg,
-                        FutureWarning,
-                        stacklevel=2,
-                    )
 
         value, method = validate_fillna_kwargs(value, method)
         if method is not None:
@@ -7559,22 +7541,6 @@ def ffill(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
-            elif (
-                not PYPY
-                and not using_copy_on_write()
-                and self._is_view_after_cow_rules()
-            ):
-                ctr = sys.getrefcount(self)
-                ref_count = REF_COUNT
-                if isinstance(self, ABCSeries) and _check_cacher(self):
-                    # see https://github.com/pandas-dev/pandas/pull/56060#discussion_r1399245221
-                    ref_count += 1
-                if ctr <= ref_count:
-                    warnings.warn(
-                        _chained_assignment_warning_method_msg,
-                        FutureWarning,
-                        stacklevel=2,
-                    )
 
         return self._pad_or_backfill(
             "ffill",
@@ -7763,22 +7729,6 @@ def bfill(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
-            elif (
-                not PYPY
-                and not using_copy_on_write()
-                and self._is_view_after_cow_rules()
-            ):
-                ctr = sys.getrefcount(self)
-                ref_count = REF_COUNT
-                if isinstance(self, ABCSeries) and _check_cacher(self):
-                    # see https://github.com/pandas-dev/pandas/pull/56060#discussion_r1399245221
-                    ref_count += 1
-                if ctr <= ref_count:
-                    warnings.warn(
-                        _chained_assignment_warning_method_msg,
-                        FutureWarning,
-                        stacklevel=2,
-                    )
 
         return self._pad_or_backfill(
             "bfill",
@@ -7934,26 +7884,6 @@ def replace(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
-            elif (
-                not PYPY
-                and not using_copy_on_write()
-                and self._is_view_after_cow_rules()
-            ):
-                ctr = sys.getrefcount(self)
-                ref_count = REF_COUNT
-                if isinstance(self, ABCSeries) and _check_cacher(self):
-                    # in non-CoW mode, chained Series access will populate the
-                    # `_item_cache` which results in an increased ref count not below
-                    # the threshold, while we still need to warn. We detect this case
-                    # of a Series derived from a DataFrame through the presence of
-                    # checking the `_cacher`
-                    ref_count += 1
-                if ctr <= ref_count:
-                    warnings.warn(
-                        _chained_assignment_warning_method_msg,
-                        FutureWarning,
-                        stacklevel=2,
-                    )
 
         if not is_bool(regex) and to_replace is not None:
             raise ValueError("'to_replace' must be 'None' if 'regex' is not a bool")
@@ -8384,22 +8314,6 @@ def interpolate(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
-            elif (
-                not PYPY
-                and not using_copy_on_write()
-                and self._is_view_after_cow_rules()
-            ):
-                ctr = sys.getrefcount(self)
-                ref_count = REF_COUNT
-                if isinstance(self, ABCSeries) and _check_cacher(self):
-                    # see https://github.com/pandas-dev/pandas/pull/56060#discussion_r1399245221
-                    ref_count += 1
-                if ctr <= ref_count:
-                    warnings.warn(
-                        _chained_assignment_warning_method_msg,
-                        FutureWarning,
-                        stacklevel=2,
-                    )
 
         axis = self._get_axis_number(axis)
 
@@ -10569,7 +10483,6 @@ def _where(
         inplace: bool_t = False,
         axis: Axis | None = None,
         level=None,
-        warn: bool_t = True,
     ):
         """
         Equivalent to public method `where`, except that `other` is not
@@ -10700,7 +10613,7 @@ def _where(
             # we may have different type blocks come out of putmask, so
             # reconstruct the block manager
 
-            new_data = self._mgr.putmask(mask=cond, new=other, align=align, warn=warn)
+            new_data = self._mgr.putmask(mask=cond, new=other, align=align)
             result = self._constructor_from_mgr(new_data, axes=new_data.axes)
             return self._update_inplace(result)
 
@@ -12566,29 +12479,8 @@ def _inplace_method(self, other, op) -> Self:
         """
         Wrap arithmetic method to operate inplace.
         """
-        warn = True
-        if not PYPY and warn_copy_on_write():
-            if sys.getrefcount(self) <= REF_COUNT + 2:
-                # we are probably in an inplace setitem context (e.g. df['a'] += 1)
-                warn = False
-
         result = op(self, other)
 
-        if (
-            self.ndim == 1
-            and result._indexed_same(self)
-            and result.dtype == self.dtype
-            and not using_copy_on_write()
-            and not (warn_copy_on_write() and not warn)
-        ):
-            # GH#36498 this inplace op can _actually_ be inplace.
-            # Item "BlockManager" of "Union[BlockManager, SingleBlockManager]" has
-            # no attribute "setitem_inplace"
-            self._mgr.setitem_inplace(  # type: ignore[union-attr]
-                slice(None), result._values, warn=warn
-            )
-            return self
-
         # Delete cacher
         self._reset_cacher()
 

pandas/core/groupby/grouper.py

@@ -12,10 +12,7 @@
 
 import numpy as np
 
-from pandas._config import (
-    using_copy_on_write,
-    warn_copy_on_write,
-)
+from pandas._config import using_copy_on_write
 
 from pandas._libs.tslibs import OutOfBoundsDatetime
 from pandas.errors import InvalidIndexError
@@ -962,7 +959,7 @@ def is_in_axis(key) -> bool:
     def is_in_obj(gpr) -> bool:
         if not hasattr(gpr, "name"):
             return False
-        if using_copy_on_write() or warn_copy_on_write():
+        if using_copy_on_write():
             # For the CoW case, we check the references to determine if the
             # series is part of the object
             try:

pandas/core/indexing.py

@@ -13,10 +13,7 @@
 
 import numpy as np
 
-from pandas._config import (
-    using_copy_on_write,
-    warn_copy_on_write,
-)
+from pandas._config import using_copy_on_write
 
 from pandas._libs.indexing import NDFrameIndexerBase
 from pandas._libs.lib import item_from_zerodim
@@ -28,11 +25,7 @@
     InvalidIndexError,
     LossySetitemError,
 )
-from pandas.errors.cow import (
-    _chained_assignment_msg,
-    _chained_assignment_warning_msg,
-    _check_cacher,
-)
+from pandas.errors.cow import _chained_assignment_msg
 from pandas.util._decorators import doc
 from pandas.util._exceptions import find_stack_level
 
@@ -889,16 +882,6 @@ def __setitem__(self, key, value) -> None:
                 warnings.warn(
                     _chained_assignment_msg, ChainedAssignmentError, stacklevel=2
                 )
-        elif not PYPY and not using_copy_on_write():
-            ctr = sys.getrefcount(self.obj)
-            ref_count = 2
-            if not warn_copy_on_write() and _check_cacher(self.obj):
-                # see https://github.com/pandas-dev/pandas/pull/56060#discussion_r1399245221
-                ref_count += 1
-            if ctr <= ref_count:
-                warnings.warn(
-                    _chained_assignment_warning_msg, FutureWarning, stacklevel=2
-                )
 
         check_dict_or_set_indexers(key)
         if isinstance(key, tuple):