CoW: Warn for inplace replace (#56297)

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

pandas/core/internals/base.py

@@ -252,12 +252,16 @@ def replace(self, to_replace, value, inplace: bool) -> Self:
             value=value,
             inplace=inplace,
             using_cow=using_copy_on_write(),
+            already_warned=_AlreadyWarned(),
         )
 
     @final
     def replace_regex(self, **kwargs) -> Self:
         return self.apply_with_block(
-            "_replace_regex", **kwargs, using_cow=using_copy_on_write()
+            "_replace_regex",
+            **kwargs,
+            using_cow=using_copy_on_write(),
+            already_warned=_AlreadyWarned(),
         )
 
     @final
@@ -278,6 +282,7 @@ def replace_list(
             inplace=inplace,
             regex=regex,
             using_cow=using_copy_on_write(),
+            already_warned=_AlreadyWarned(),
         )
         bm._consolidate_inplace()
         return bm

pandas/core/internals/blocks.py

@@ -830,6 +830,7 @@ def replace(
         # mask may be pre-computed if we're called from replace_list
         mask: npt.NDArray[np.bool_] | None = None,
         using_cow: bool = False,
+        already_warned=None,
     ) -> list[Block]:
         """
         replace the to_replace value with value, possible to create new
@@ -874,6 +875,20 @@ def replace(
             # and rest?
             blk = self._maybe_copy(using_cow, inplace)
             putmask_inplace(blk.values, mask, value)
+            if (
+                inplace
+                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
+
             if not (self.is_object and value is None):
                 # if the user *explicitly* gave None, we keep None, otherwise
                 #  may downcast to NaN
@@ -934,6 +949,7 @@ def _replace_regex(
         inplace: bool = False,
         mask=None,
         using_cow: bool = False,
+        already_warned=None,
     ) -> list[Block]:
         """
         Replace elements by the given value.
@@ -968,6 +984,20 @@ def _replace_regex(
 
         replace_regex(block.values, rx, value, mask)
 
+        if (
+            inplace
+            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
+
         nbs = block.convert(copy=False, using_cow=using_cow)
         opt = get_option("future.no_silent_downcasting")
         if (len(nbs) > 1 or nbs[0].dtype != block.dtype) and not opt:
@@ -992,6 +1022,7 @@ def replace_list(
         inplace: bool = False,
         regex: bool = False,
         using_cow: bool = False,
+        already_warned=None,
     ) -> list[Block]:
         """
         See BlockManager.replace_list docstring.
@@ -1048,6 +1079,20 @@ def replace_list(
         else:
             rb = [self if inplace else self.copy()]
 
+        if (
+            inplace
+            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
+
         opt = get_option("future.no_silent_downcasting")
         for i, ((src, dest), mask) in enumerate(zip(pairs, masks)):
             convert = i == src_len  # only convert once at the end

pandas/tests/copy_view/test_chained_assignment_deprecation.py

@@ -45,12 +45,9 @@ def test_methods_iloc_warn(using_copy_on_write):
 def test_methods_iloc_getitem_item_cache(
     func, args, using_copy_on_write, warn_copy_on_write
 ):
-    df = DataFrame({"a": [1.5, 2, 3], "b": 1.5})
+    df = DataFrame({"a": [1, 2, 3], "b": 1})
     ser = df.iloc[:, 0]
-    # TODO(CoW-warn) should warn about updating a view for all methods
-    with tm.assert_cow_warning(
-        warn_copy_on_write and func not in ("replace", "fillna")
-    ):
+    with tm.assert_cow_warning(warn_copy_on_write and func == "replace"):
         getattr(ser, func)(*args, inplace=True)
 
     # parent that holds item_cache is dead, so don't increase ref count

pandas/tests/copy_view/test_replace.py

@@ -48,12 +48,13 @@ def test_replace(using_copy_on_write, replace_kwargs):
     tm.assert_frame_equal(df, df_orig)
 
 
-def test_replace_regex_inplace_refs(using_copy_on_write):
+def test_replace_regex_inplace_refs(using_copy_on_write, warn_copy_on_write):
     df = DataFrame({"a": ["aaa", "bbb"]})
     df_orig = df.copy()
     view = df[:]
     arr = get_array(df, "a")
-    df.replace(to_replace=r"^a.*$", value="new", inplace=True, regex=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace(to_replace=r"^a.*$", value="new", inplace=True, regex=True)
     if using_copy_on_write:
         assert not np.shares_memory(arr, get_array(df, "a"))
         assert df._mgr._has_no_reference(0)
@@ -202,11 +203,12 @@ def test_replace_inplace(using_copy_on_write, to_replace):
 
 
 @pytest.mark.parametrize("to_replace", [1.5, [1.5]])
-def test_replace_inplace_reference(using_copy_on_write, to_replace):
+def test_replace_inplace_reference(using_copy_on_write, to_replace, warn_copy_on_write):
     df = DataFrame({"a": [1.5, 2, 3]})
     arr_a = get_array(df, "a")
     view = df[:]
-    df.replace(to_replace=to_replace, value=15.5, inplace=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace(to_replace=to_replace, value=15.5, inplace=True)
 
     if using_copy_on_write:
         assert not np.shares_memory(get_array(df, "a"), arr_a)
@@ -354,12 +356,13 @@ def test_replace_list_none(using_copy_on_write):
     assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
 
 
-def test_replace_list_none_inplace_refs(using_copy_on_write):
+def test_replace_list_none_inplace_refs(using_copy_on_write, warn_copy_on_write):
     df = DataFrame({"a": ["a", "b", "c"]})
     arr = get_array(df, "a")
     df_orig = df.copy()
     view = df[:]
-    df.replace(["a"], value=None, inplace=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace(["a"], value=None, inplace=True)
     if using_copy_on_write:
         assert df._mgr._has_no_reference(0)
         assert not np.shares_memory(arr, get_array(df, "a"))
@@ -431,15 +434,16 @@ def test_replace_listlike(using_copy_on_write):
     tm.assert_frame_equal(df, df_orig)
 
 
-def test_replace_listlike_inplace(using_copy_on_write):
+def test_replace_listlike_inplace(using_copy_on_write, warn_copy_on_write):
     df = DataFrame({"a": [1, 2, 3], "b": [1, 2, 3]})
     arr = get_array(df, "a")
     df.replace([200, 2], [10, 11], inplace=True)
     assert np.shares_memory(get_array(df, "a"), arr)
 
     view = df[:]
     df_orig = df.copy()
-    df.replace([200, 3], [10, 11], inplace=True)
+    with tm.assert_cow_warning(warn_copy_on_write):
+        df.replace([200, 3], [10, 11], inplace=True)
     if using_copy_on_write:
         assert not np.shares_memory(get_array(df, "a"), arr)
         tm.assert_frame_equal(view, df_orig)