CoW: Add warning for fillna with inplace

pandas/core/generic.py

@@ -7180,6 +7180,18 @@ def fillna(
                         ChainedAssignmentError,
                         stacklevel=2,
                     )
+            elif not PYPY and not using_copy_on_write():
+                ctr = sys.getrefcount(self)
+                ref_count = REF_COUNT
+                if isinstance(self, ABCSeries) and hasattr(self, "_cacher"):
+                    # 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:

pandas/tests/copy_view/test_interp_fillna.py

@@ -10,6 +10,7 @@
     Series,
     Timestamp,
     interval_range,
+    option_context,
 )
 import pandas._testing as tm
 from pandas.tests.copy_view.util import get_array
@@ -364,6 +365,17 @@ def test_fillna_chained_assignment(using_copy_on_write):
         with tm.raises_chained_assignment_error():
             df[["a"]].fillna(100, inplace=True)
         tm.assert_frame_equal(df, df_orig)
+    else:
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            with option_context("mode.chained_assignment", None):
+                df[["a"]].fillna(100, inplace=True)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            with option_context("mode.chained_assignment", None):
+                df[df.a > 5].fillna(100, inplace=True)
+
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].fillna(100, inplace=True)
 
 
 @pytest.mark.parametrize("func", ["interpolate", "ffill", "bfill"])

pandas/tests/frame/methods/test_fillna.py

@@ -58,7 +58,8 @@ def test_fillna_on_column_view(self, using_copy_on_write):
                 df[0].fillna(-1, inplace=True)
             assert np.isnan(arr[:, 0]).all()
         else:
-            df[0].fillna(-1, inplace=True)
+            with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+                df[0].fillna(-1, inplace=True)
             assert (arr[:, 0] == -1).all()
 
         # i.e. we didn't create a new 49-column block

pandas/tests/frame/test_block_internals.py

@@ -429,7 +429,8 @@ def test_update_inplace_sets_valid_block_values(using_copy_on_write):
         with tm.raises_chained_assignment_error():
             df["a"].fillna(1, inplace=True)
     else:
-        df["a"].fillna(1, inplace=True)
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            df["a"].fillna(1, inplace=True)
 
     # check we haven't put a Series into any block.values
     assert isinstance(df._mgr.blocks[0].values, Categorical)

pandas/tests/indexing/multiindex/test_chaining_and_caching.py

@@ -34,12 +34,13 @@ def test_detect_chained_assignment(using_copy_on_write, warn_copy_on_write):
         with tm.raises_chained_assignment_error():
             zed["eyes"]["right"].fillna(value=555, inplace=True)
     elif warn_copy_on_write:
-        # TODO(CoW-warn) should warn
-        zed["eyes"]["right"].fillna(value=555, inplace=True)
+        with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+            zed["eyes"]["right"].fillna(value=555, inplace=True)
     else:
         msg = "A value is trying to be set on a copy of a slice from a DataFrame"
         with pytest.raises(SettingWithCopyError, match=msg):
-            zed["eyes"]["right"].fillna(value=555, inplace=True)
+            with tm.assert_produces_warning(FutureWarning, match="inplace method"):
+                zed["eyes"]["right"].fillna(value=555, inplace=True)
 
 
 @td.skip_array_manager_invalid_test  # with ArrayManager df.loc[0] is not a view