CoW: Delay copy when inserting Series into DataFrame (pandas-dev#52061)

Author: phofl

pandas/core/frame.py

@@ -4819,8 +4819,8 @@ def insert(
         elif isinstance(value, DataFrame):
             value = value.iloc[:, 0]
 
-        value, _ = self._sanitize_column(value)
-        self._mgr.insert(loc, column, value)
+        value, refs = self._sanitize_column(value, using_cow=using_copy_on_write())
+        self._mgr.insert(loc, column, value, refs=refs)
 
     def assign(self, **kwargs) -> DataFrame:
         r"""

pandas/tests/copy_view/test_methods.py

@@ -1739,3 +1739,18 @@ def test_series_view(using_copy_on_write):
     else:
         expected = Series([100, 2, 3])
         tm.assert_series_equal(ser, expected)
+
+
+def test_insert_series(using_copy_on_write):
+    df = DataFrame({"a": [1, 2, 3]})
+    ser = Series([1, 2, 3])
+    ser_orig = ser.copy()
+    df.insert(loc=1, value=ser, column="b")
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(ser), get_array(df, "b"))
+        assert not df._mgr._has_no_reference(1)
+    else:
+        assert not np.shares_memory(get_array(ser), get_array(df, "b"))
+
+    df.iloc[0, 1] = 100
+    tm.assert_series_equal(ser, ser_orig)