BUG-CoW: DataFrame constructed from Series not respecting CoW

doc/source/whatsnew/v2.0.0.rst

@@ -190,6 +190,9 @@ Copy-on-Write improvements
   of Series objects and specifying ``copy=False``, will now use a lazy copy
   of those Series objects for the columns of the DataFrame (:issue:`50777`)
 
+- The :class:`DataFrame` constructor, when constructing a DataFrame from a
+  :class:`Series` and specifying ``copy=False``, will now respect Copy-on-Write.
+
 - The :class:`DataFrame` constructor, when constructing from a NumPy array,
   will now copy the array by default to avoid mutating the :class:`DataFrame`
   when mutating the array. Specify ``copy=False`` to get the old behavior.

pandas/core/internals/construction.py

@@ -17,6 +17,7 @@
 
 from pandas._libs import lib
 
+from pandas.core.dtypes.astype import astype_is_view
 from pandas.core.dtypes.cast import (
     construct_1d_arraylike_from_scalar,
     dict_compat,
@@ -260,6 +261,7 @@ def ndarray_to_mgr(
     copy_on_sanitize = False if typ == "array" else copy
 
     vdtype = getattr(values, "dtype", None)
+    refs = None
     if is_1d_only_ea_dtype(vdtype) or is_1d_only_ea_dtype(dtype):
         # GH#19157
 
@@ -291,7 +293,20 @@ def ndarray_to_mgr(
         if values.ndim == 1:
             values = values.reshape(-1, 1)
 
-    elif isinstance(values, (np.ndarray, ExtensionArray, ABCSeries, Index)):
+    elif isinstance(values, ABCSeries):
+        if not copy_on_sanitize and (
+            dtype is None or astype_is_view(values.dtype, dtype)
+        ):
+            refs = values._references
+
+        if copy_on_sanitize:
+            values = values._values.copy()
+        else:
+            values = values._values
+
+        values = _ensure_2d(values)
+
+    elif isinstance(values, (np.ndarray, ExtensionArray, Index)):
         # drop subclass info
         values = np.array(values, copy=copy_on_sanitize)
         values = _ensure_2d(values)
@@ -356,11 +371,11 @@ def ndarray_to_mgr(
             ]
         else:
             bp = BlockPlacement(slice(len(columns)))
-            nb = new_block_2d(values, placement=bp)
+            nb = new_block_2d(values, placement=bp, refs=refs)
             block_values = [nb]
     else:
         bp = BlockPlacement(slice(len(columns)))
-        nb = new_block_2d(values, placement=bp)
+        nb = new_block_2d(values, placement=bp, refs=refs)
         block_values = [nb]
 
     if len(columns) == 0:

pandas/tests/copy_view/test_constructors.py

@@ -200,6 +200,38 @@ def test_dataframe_from_dict_of_series_with_reindex(dtype):
     assert np.shares_memory(arr_before, arr_after)
 
 
+@pytest.mark.parametrize(
+    "data, dtype", [([1, 2], None), ([1, 2], "int64"), (["a", "b"], None)]
+)
+def test_dataframe_from_series(using_copy_on_write, data, dtype):
+    ser = Series(data, dtype=dtype)
+    ser_orig = ser.copy()
+    df = DataFrame(ser, dtype=dtype)
+    assert np.shares_memory(get_array(ser), get_array(df, 0))
+    if using_copy_on_write:
+        assert not df._mgr._has_no_reference(0)
+
+    df.iloc[0, 0] = data[-1]
+    if using_copy_on_write:
+        tm.assert_series_equal(ser, ser_orig)
+
+
+def test_dataframe_from_series_different_dtype(using_copy_on_write):
+    ser = Series([1, 2], dtype="int64")
+    df = DataFrame(ser, dtype="int32")
+    assert not np.shares_memory(get_array(ser), get_array(df, 0))
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
+def test_dataframe_from_series_infer_datetime(using_copy_on_write):
+    ser = Series([Timestamp("2019-12-31"), Timestamp("2020-12-31")], dtype=object)
+    df = DataFrame(ser)
+    assert not np.shares_memory(get_array(ser), get_array(df, 0))
+    if using_copy_on_write:
+        assert df._mgr._has_no_reference(0)
+
+
 @pytest.mark.parametrize("index", [None, [0, 1, 2]])
 def test_dataframe_from_dict_of_series_with_dtype(index):
     # Variant of above, but now passing a dtype that causes a copy