ENH: Improve replace lazy copy handling

pandas/core/internals/blocks.py

@@ -714,6 +714,8 @@ def replace_list(
             (x, y) for x, y in zip(src_list, dest_list) if self._can_hold_element(x)
         ]
         if not len(pairs):
+            if using_cow:
+                return [self.copy(deep=False)]
             # shortcut, nothing to replace
             return [self] if inplace else [self.copy()]
 
@@ -734,8 +736,9 @@ def replace_list(
         masks = [extract_bool_array(x) for x in masks]
 
         if using_cow and inplace:
-            # TODO(CoW): Optimize
-            rb = [self.copy()]
+            # Don't set up refs here, otherwise we will think that we have
+            # references when we check again later
+            rb = [self]
         else:
             rb = [self if inplace else self.copy()]
         for i, (src, dest) in enumerate(pairs):
@@ -762,10 +765,16 @@ def replace_list(
                     mask=m,  # type: ignore[arg-type]
                     inplace=inplace,
                     regex=regex,
+                    using_cow=using_cow,
                 )
                 if convert and blk.is_object and not all(x is None for x in dest_list):
                     # GH#44498 avoid unwanted cast-back
-                    result = extend_blocks([b.convert(copy=True) for b in result])
+                    result = extend_blocks(
+                        [
+                            b.convert(copy=True and not using_cow, using_cow=using_cow)
+                            for b in result
+                        ]
+                    )
                 new_rb.extend(result)
             rb = new_rb
         return rb
@@ -778,6 +787,7 @@ def _replace_coerce(
         mask: npt.NDArray[np.bool_],
         inplace: bool = True,
         regex: bool = False,
+        using_cow: bool = False,
     ) -> list[Block]:
         """
         Replace value corresponding to the given boolean array with another
@@ -811,17 +821,24 @@ def _replace_coerce(
             if value is None:
                 # gh-45601, gh-45836, gh-46634
                 if mask.any():
-                    nb = self.astype(np.dtype(object), copy=False)
-                    if nb is self and not inplace:
+                    has_ref = self.refs.has_reference()
+                    nb = self.astype(np.dtype(object), copy=False, using_cow=using_cow)
+                    if (nb is self or using_cow) and not inplace:
+                        nb = nb.copy()
+                    elif inplace and has_ref and nb.refs.has_reference():
+                        # no copy in astype and we had refs before
                         nb = nb.copy()
                     putmask_inplace(nb.values, mask, value)
                     return [nb]
+                if using_cow:
+                    return [self.copy(deep=False)]
                 return [self] if inplace else [self.copy()]
             return self.replace(
                 to_replace=to_replace,
                 value=value,
                 inplace=inplace,
                 mask=mask,
+                using_cow=using_cow,
             )
 
     # ---------------------------------------------------------------------

pandas/tests/copy_view/test_replace.py

@@ -112,7 +112,8 @@ def test_replace_to_replace_wrong_dtype(using_copy_on_write):
         assert not np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
 
 
-def test_replace_inplace(using_copy_on_write):
+@pytest.mark.parametrize("to_replace", [1.5, [1.5], []])
+def test_replace_inplace(using_copy_on_write, to_replace):
     df = DataFrame({"a": [1.5, 2, 3]})
     arr_a = get_array(df, "a")
     df.replace(to_replace=1.5, value=15.5, inplace=True)
@@ -122,17 +123,22 @@ def test_replace_inplace(using_copy_on_write):
         assert df._mgr._has_no_reference(0)
 
 
-@pytest.mark.parametrize("to_replace", [1.5, [1.5]])
+@pytest.mark.parametrize("to_replace", [1.5, [1.5], []])
 def test_replace_inplace_reference(using_copy_on_write, to_replace):
     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)
 
     if using_copy_on_write:
-        assert not np.shares_memory(get_array(df, "a"), arr_a)
-        assert df._mgr._has_no_reference(0)
-        assert view._mgr._has_no_reference(0)
+        if isinstance(to_replace, list) and len(to_replace) == 0:
+            assert np.shares_memory(get_array(df, "a"), arr_a)
+            assert not df._mgr._has_no_reference(0)
+            assert not view._mgr._has_no_reference(0)
+        else:
+            assert not np.shares_memory(get_array(df, "a"), arr_a)
+            assert df._mgr._has_no_reference(0)
+            assert view._mgr._has_no_reference(0)
     else:
         assert np.shares_memory(get_array(df, "a"), arr_a)
 
@@ -216,3 +222,63 @@ def test_masking_inplace(using_copy_on_write, method):
         tm.assert_frame_equal(view, df_orig)
     else:
         assert np.shares_memory(get_array(df, "a"), arr_a)
+
+
+def test_replace_empty_list(using_copy_on_write):
+    df = DataFrame({"a": [1, 2]})
+
+    df2 = df.replace([], [])
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+        assert not df._mgr._has_no_reference(0)
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+
+@pytest.mark.parametrize("value", ["d", None])
+def test_replace_object_list_inplace(using_copy_on_write, value):
+    df = DataFrame({"a": ["a", "b", "c"]})
+    arr = get_array(df, "a")
+    df.replace(["c"], value, inplace=True)
+    if using_copy_on_write or value is None:
+        assert np.shares_memory(arr, get_array(df, "a"))
+        assert df._mgr._has_no_reference(0)
+    else:
+        # This could be inplace
+        assert not np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_replace_list_multiple_elements_inplace(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]})
+    arr = get_array(df, "a")
+    df.replace([1, 2], 4, inplace=True)
+    if using_copy_on_write:
+        # TODO(CoW): This should share memory
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        assert df._mgr._has_no_reference(0)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a"))
+
+
+def test_replace_list_none(using_copy_on_write):
+    df = DataFrame({"a": ["a", "b", "c"]})
+
+    df_orig = df.copy()
+    df2 = df.replace(["b"], value=None)
+    tm.assert_frame_equal(df, df_orig)
+
+    assert not np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+
+def test_replace_list_none_inplace_refs(using_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)
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+        assert not np.shares_memory(arr, get_array(df, "a"))
+        tm.assert_frame_equal(df_orig, view)
+    else:
+        assert np.shares_memory(arr, get_array(df, "a"))