CoW: Add warning for fillna (#56288)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: phofl

pandas/core/internals/base.py

@@ -190,6 +190,7 @@ def fillna(self, value, limit: int | None, inplace: bool, downcast) -> Self:
             inplace=inplace,
             downcast=downcast,
             using_cow=using_copy_on_write(),
+            already_warned=_AlreadyWarned(),
         )
 
     @final

pandas/core/internals/blocks.py

@@ -1591,6 +1591,7 @@ def fillna(
         inplace: bool = False,
         downcast=None,
         using_cow: bool = False,
+        already_warned=None,
     ) -> list[Block]:
         """
         fillna on the block with the value. If we fail, then convert to
@@ -1626,7 +1627,9 @@ def fillna(
             mask[mask.cumsum(self.ndim - 1) > limit] = False
 
         if inplace:
-            nbs = self.putmask(mask.T, value, using_cow=using_cow)
+            nbs = self.putmask(
+                mask.T, value, using_cow=using_cow, already_warned=already_warned
+            )
         else:
             # without _downcast, we would break
             #  test_fillna_dtype_conversion_equiv_replace
@@ -2208,6 +2211,7 @@ def fillna(
         inplace: bool = False,
         downcast=None,
         using_cow: bool = False,
+        already_warned=None,
     ) -> list[Block]:
         if isinstance(self.dtype, IntervalDtype):
             # Block.fillna handles coercion (test_fillna_interval)
@@ -2217,6 +2221,7 @@ def fillna(
                 inplace=inplace,
                 downcast=downcast,
                 using_cow=using_cow,
+                already_warned=already_warned,
             )
         if using_cow and self._can_hold_na and not self.values._hasna:
             refs = self.refs
@@ -2244,6 +2249,20 @@ def fillna(
                     DeprecationWarning,
                     stacklevel=find_stack_level(),
                 )
+            else:
+                if (
+                    not copy
+                    and warn_copy_on_write()
+                    and already_warned is not None
+                    and not already_warned.warned_already
+                ):
+                    if self.refs.has_reference():
+                        warnings.warn(
+                            COW_WARNING_GENERAL_MSG,
+                            FutureWarning,
+                            stacklevel=find_stack_level(),
+                        )
+                        already_warned.warned_already = True
 
         nb = self.make_block_same_class(new_values, refs=refs)
         return nb._maybe_downcast([nb], downcast, using_cow=using_cow, caller="fillna")

pandas/tests/copy_view/test_interp_fillna.py

@@ -229,14 +229,15 @@ def test_fillna_inplace(using_copy_on_write, downcast):
         assert df._mgr._has_no_reference(1)
 
 
-def test_fillna_inplace_reference(using_copy_on_write):
+def test_fillna_inplace_reference(using_copy_on_write, warn_copy_on_write):
     df = DataFrame({"a": [1.5, np.nan], "b": 1})
     df_orig = df.copy()
     arr_a = get_array(df, "a")
     arr_b = get_array(df, "b")
     view = df[:]
 
-    df.fillna(5.5, inplace=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.fillna(5.5, inplace=True)
     if using_copy_on_write:
         assert not np.shares_memory(get_array(df, "a"), arr_a)
         assert np.shares_memory(get_array(df, "b"), arr_b)
@@ -250,7 +251,7 @@ def test_fillna_inplace_reference(using_copy_on_write):
     tm.assert_frame_equal(df, expected)
 
 
-def test_fillna_interval_inplace_reference(using_copy_on_write):
+def test_fillna_interval_inplace_reference(using_copy_on_write, warn_copy_on_write):
     # Set dtype explicitly to avoid implicit cast when setting nan
     ser = Series(
         interval_range(start=0, end=5), name="a", dtype="interval[float64, right]"
@@ -259,7 +260,8 @@ def test_fillna_interval_inplace_reference(using_copy_on_write):
 
     ser_orig = ser.copy()
     view = ser[:]
-    ser.fillna(value=Interval(left=0, right=5), inplace=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        ser.fillna(value=Interval(left=0, right=5), inplace=True)
 
     if using_copy_on_write:
         assert not np.shares_memory(
@@ -330,8 +332,8 @@ def test_fillna_inplace_ea_noop_shares_memory(
     df = DataFrame({"a": [1, NA, 3], "b": 1}, dtype=any_numeric_ea_and_arrow_dtype)
     df_orig = df.copy()
     view = df[:]
-    # TODO(CoW-warn)
-    df.fillna(100, inplace=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.fillna(100, inplace=True)
 
     if isinstance(df["a"].dtype, ArrowDtype) or using_copy_on_write:
         assert not np.shares_memory(get_array(df, "a"), get_array(view, "a"))

pandas/tests/frame/methods/test_fillna.py

@@ -745,8 +745,7 @@ def test_inplace_dict_update_view(
         df = DataFrame({"x": [np.nan, 2], "y": [np.nan, 2]})
         df_orig = df.copy()
         result_view = df[:]
-        # TODO(CoW-warn) better warning message + should warn in all cases
-        with tm.assert_cow_warning(warn_copy_on_write and not isinstance(val, int)):
+        with tm.assert_cow_warning(warn_copy_on_write):
             df.fillna(val, inplace=True)
         expected = DataFrame({"x": [-1, 2.0], "y": [-1.0, 2]})
         tm.assert_frame_equal(df, expected)