avoid warning with inplace operators

Author: jorisvandenbossche

pandas/core/generic.py

@@ -12375,14 +12375,20 @@ 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) <= 5:
+                # 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:
             # GH#36498 this inplace op can _actually_ be inplace.
             # Item "ArrayManager" of "Union[ArrayManager, SingleArrayManager,
             # BlockManager, SingleBlockManager]" has no attribute "setitem_inplace"
             self._mgr.setitem_inplace(  # type: ignore[union-attr]
-                slice(None), result._values
+                slice(None), result._values, warn=warn
             )
             return self
 

pandas/core/internals/base.py

@@ -307,7 +307,7 @@ def array(self) -> ArrayLike:
         # error: "SingleDataManager" has no attribute "arrays"; maybe "array"
         return self.arrays[0]  # type: ignore[attr-defined]
 
-    def setitem_inplace(self, indexer, value) -> None:
+    def setitem_inplace(self, indexer, value, warn: bool = True) -> None:
         """
         Set values with indexer.
 

pandas/core/internals/managers.py

@@ -1981,7 +1981,7 @@ def get_numeric_data(self, copy: bool = False) -> Self:
     def _can_hold_na(self) -> bool:
         return self._block._can_hold_na
 
-    def setitem_inplace(self, indexer, value) -> None:
+    def setitem_inplace(self, indexer, value, warn: bool = True) -> None:
         """
         Set values with indexer.
 
@@ -1995,7 +1995,7 @@ def setitem_inplace(self, indexer, value) -> None:
             if using_copy_on_write():
                 self.blocks = (self._block.copy(),)
                 self._cache.clear()
-            elif warn_copy_on_write():
+            elif warn and warn_copy_on_write():
                 warnings.warn(
                     "Setting value on view: behaviour will change in pandas 3.0 "
                     "with Copy-on-Write ...",

pandas/tests/copy_view/test_setitem.py

@@ -140,3 +140,19 @@ def test_setitem_series_column_midx_broadcasting(using_copy_on_write):
     assert not np.shares_memory(get_array(rhs), df._get_column_array(0))
     if using_copy_on_write:
         assert df._mgr._has_no_reference(0)
+
+
+def test_set_column_with_inplace_operator(using_copy_on_write, warn_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+
+    # this should not raise any warning
+    with tm.assert_produces_warning(None):
+        df["a"] += 1
+
+    # when it is not in a chain, then it should produce a warning
+    warn = FutureWarning if warn_copy_on_write else None
+
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    ser = df["a"]
+    with tm.assert_produces_warning(warn):
+        ser += 1

pandas/tests/frame/test_arithmetic.py

@@ -2029,14 +2029,16 @@ def test_arith_list_of_arraylike_raise(to_add):
         to_add + df
 
 
-def test_inplace_arithmetic_series_update(using_copy_on_write):
+def test_inplace_arithmetic_series_update(using_copy_on_write, warn_copy_on_write):
     # https://github.com/pandas-dev/pandas/issues/36373
     df = DataFrame({"A": [1, 2, 3]})
     df_orig = df.copy()
     series = df["A"]
     vals = series._values
 
-    series += 1
+    warn = FutureWarning if warn_copy_on_write else None
+    with tm.assert_produces_warning(warn):
+        series += 1
     if using_copy_on_write:
         assert series._values is not vals
         tm.assert_frame_equal(df, df_orig)

pandas/tests/frame/test_subclass.py

@@ -520,6 +520,8 @@ def test_subclassed_wide_to_long(self):
 
         tm.assert_frame_equal(long_frame, expected)
 
+    # TODO(CoW) should not need to warn
+    @pytest.mark.filterwarnings("ignore:Setting value on view:FutureWarning")
     def test_subclassed_apply(self):
         # GH 19822