ENH: Make shallow copy for align nocopy with CoW

pandas/core/generic.py

@@ -5429,6 +5429,8 @@ def _reindex_with_indexers(
 
         if (copy or copy is None) and new_data is self._mgr:
             new_data = new_data.copy(deep=copy)
+        elif using_copy_on_write() and new_data is self._mgr:
+            new_data = new_data.copy(deep=copy)
 
         return self._constructor(new_data).__finalize__(self)
 
@@ -9469,6 +9471,7 @@ def _align_series(
         limit=None,
         fill_axis: Axis = 0,
     ):
+        uses_cow = using_copy_on_write()
 
         is_series = isinstance(self, ABCSeries)
 
@@ -9492,7 +9495,10 @@ def _align_series(
             if is_series:
                 left = self._reindex_indexer(join_index, lidx, copy)
             elif lidx is None or join_index is None:
-                left = self.copy(deep=copy) if copy or copy is None else self
+                if uses_cow:
+                    left = self.copy(deep=copy)
+                else:
+                    left = self.copy(deep=copy) if copy or copy is None else self
             else:
                 left = self._constructor(
                     self._mgr.reindex_indexer(join_index, lidx, axis=1, copy=copy)
@@ -9521,7 +9527,10 @@ def _align_series(
             left = self._constructor(fdata)
 
             if ridx is None:
-                right = other.copy(deep=copy) if copy or copy is None else other
+                if uses_cow:
+                    right = other.copy(deep=copy)
+                else:
+                    right = other.copy(deep=copy) if copy or copy is None else other
             else:
                 right = other.reindex(join_index, level=level)
 

pandas/core/series.py

@@ -4643,6 +4643,8 @@ def _reindex_indexer(
         if indexer is None and (
             new_index is None or new_index.names == self.index.names
         ):
+            if using_copy_on_write():
+                return self.copy(deep=copy)
             if copy or copy is None:
                 return self.copy(deep=copy)
             return self

pandas/tests/copy_view/test_methods.py

@@ -456,6 +456,41 @@ def test_align_series(using_copy_on_write):
     tm.assert_series_equal(ser_other, ser_orig)
 
 
+def test_align_copy_false(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    df_orig = df.copy()
+    df2, df3 = df.align(df, copy=False)
+
+    assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+    assert np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+
+    if using_copy_on_write:
+        df2.loc[0, "a"] = 0
+        tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+
+        df3.loc[0, "a"] = 0
+        tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+
+
+def test_align_with_series_copy_false(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    ser = Series([1, 2, 3], name="x")
+    ser_orig = ser.copy()
+    df_orig = df.copy()
+    df2, ser2 = df.align(ser, copy=False, axis=0)
+
+    assert np.shares_memory(get_array(df, "b"), get_array(df2, "b"))
+    assert np.shares_memory(get_array(df, "a"), get_array(df2, "a"))
+    assert np.shares_memory(get_array(ser, "x"), get_array(ser2, "x"))
+
+    if using_copy_on_write:
+        df2.loc[0, "a"] = 0
+        tm.assert_frame_equal(df, df_orig)  # Original is unchanged
+
+        ser2.loc[0] = 0
+        tm.assert_series_equal(ser, ser_orig)  # Original is unchanged
+
+
 def test_to_frame(using_copy_on_write):
     # Case: converting a Series to a DataFrame with to_frame
     ser = Series([1, 2, 3])

pandas/tests/frame/methods/test_align.py

@@ -40,12 +40,15 @@ def test_frame_align_aware(self):
         assert new1.index.tz is timezone.utc
         assert new2.index.tz is timezone.utc
 
-    def test_align_float(self, float_frame):
+    def test_align_float(self, float_frame, using_copy_on_write):
         af, bf = float_frame.align(float_frame)
         assert af._mgr is not float_frame._mgr
 
         af, bf = float_frame.align(float_frame, copy=False)
-        assert af._mgr is float_frame._mgr
+        if not using_copy_on_write:
+            assert af._mgr is float_frame._mgr
+        else:
+            assert af._mgr is not float_frame._mgr
 
         # axis = 0
         other = float_frame.iloc[:-5, :3]

pandas/tests/series/methods/test_align.py

@@ -82,7 +82,7 @@ def test_align_fill_method(
     tm.assert_series_equal(ab, eb)
 
 
-def test_align_nocopy(datetime_series):
+def test_align_nocopy(datetime_series, using_copy_on_write):
     b = datetime_series[:5].copy()
 
     # do copy
@@ -95,7 +95,10 @@ def test_align_nocopy(datetime_series):
     a = datetime_series.copy()
     ra, _ = a.align(b, join="left", copy=False)
     ra[:5] = 5
-    assert (a[:5] == 5).all()
+    if using_copy_on_write:
+        assert not (a[:5] == 5).any()
+    else:
+        assert (a[:5] == 5).all()
 
     # do copy
     a = datetime_series.copy()
@@ -109,7 +112,10 @@ def test_align_nocopy(datetime_series):
     b = datetime_series[:5].copy()
     _, rb = a.align(b, join="right", copy=False)
     rb[:2] = 5
-    assert (b[:2] == 5).all()
+    if using_copy_on_write:
+        assert not (b[:2] == 5).any()
+    else:
+        assert (b[:2] == 5).all()
 
 
 def test_align_same_index(datetime_series):