CoW: Copy less in replace implementation with listlikes (pandas-dev#5…

…4116)

pandas/core/internals/blocks.py

@@ -11,6 +11,7 @@
     final,
 )
 import warnings
+import weakref
 
 import numpy as np
 
@@ -839,7 +840,7 @@ def replace_list(
         if inplace:
             masks = list(masks)
 
-        if using_cow and inplace:
+        if using_cow:
             # Don't set up refs here, otherwise we will think that we have
             # references when we check again later
             rb = [self]
@@ -872,6 +873,17 @@ def replace_list(
                     regex=regex,
                     using_cow=using_cow,
                 )
+
+                if using_cow and i != src_len:
+                    # This is ugly, but we have to get rid of intermediate refs
+                    # that did not go out of scope yet, otherwise we will trigger
+                    # many unnecessary copies
+                    for b in result:
+                        ref = weakref.ref(b)
+                        b.refs.referenced_blocks.pop(
+                            b.refs.referenced_blocks.index(ref)
+                        )
+
                 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(
@@ -936,7 +948,7 @@ def _replace_coerce(
                     putmask_inplace(nb.values, mask, value)
                     return [nb]
                 if using_cow:
-                    return [self.copy(deep=False)]
+                    return [self]
                 return [self] if inplace else [self.copy()]
             return self.replace(
                 to_replace=to_replace,

pandas/tests/copy_view/test_replace.py

@@ -333,8 +333,7 @@ def test_replace_list_multiple_elements_inplace(using_copy_on_write):
     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 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"))
@@ -396,3 +395,38 @@ def test_replace_chained_assignment(using_copy_on_write):
         with tm.raises_chained_assignment_error():
             df[["a"]].replace(1, 100, inplace=True)
         tm.assert_frame_equal(df, df_orig)
+
+
+def test_replace_listlike(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [1, 2, 3]})
+    df_orig = df.copy()
+
+    result = df.replace([200, 201], [11, 11])
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(result, "a"), get_array(df, "a"))
+
+    result.iloc[0, 0] = 100
+    tm.assert_frame_equal(df, df)
+
+    result = df.replace([200, 2], [10, 10])
+    assert not np.shares_memory(get_array(df, "a"), get_array(result, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
+def test_replace_listlike_inplace(using_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)
+    if using_copy_on_write:
+        assert not np.shares_memory(get_array(df, "a"), arr)
+        tm.assert_frame_equal(view, df_orig)
+    else:
+        assert np.shares_memory(get_array(df, "a"), arr)
+        tm.assert_frame_equal(df, view)