BUG: ensure we still honor copy=True in Series constructor in all cases

pandas/core/series.py

@@ -265,8 +265,12 @@ class Series(base.IndexOpsMixin, NDFrame):  # type: ignore[misc]
         See the :ref:`user guide <basics.dtypes>` for more usages.
     name : Hashable, default None
         The name to give to the Series.
-    copy : bool, default False
-        Copy input data. Only affects Series or 1d ndarray input. See examples.
+    copy : bool, default None
+        Copy input data. By default, will copy if the input data is a numpy or
+        pandas array.
+        Set to False to avoid copying, at your own risk (if you know the input
+        data won't be modified elsewhere).
+        Only affects Series or 1d ndarray input. See examples.
 
     See Also
     --------
@@ -397,6 +401,7 @@ def __init__(
             if copy is not False:
                 if dtype is None or astype_is_view(data.dtype, pandas_dtype(dtype)):
                     data = data.copy()
+                    copy = False
         if copy is None:
             copy = False
 
@@ -411,6 +416,7 @@ def __init__(
                     Pandas4Warning,
                     stacklevel=2,
                 )
+                allow_mgr = True
 
         name = ibase.maybe_extract_name(name, data, type(self))
 
@@ -436,9 +442,8 @@ def __init__(
         if isinstance(data, Index):
             if dtype is not None:
                 data = data.astype(dtype)
-
-            refs = data._references
-            copy = False
+            if not copy:
+                refs = data._references
 
         elif isinstance(data, np.ndarray):
             if len(data.dtype):
@@ -454,8 +459,9 @@ def __init__(
                 data = data._mgr.copy(deep=False)
             else:
                 data = data.reindex(index)
-                copy = False
                 data = data._mgr
+                if data._has_no_reference(0):
+                    copy = False
         elif isinstance(data, Mapping):
             data, index = self._init_dict(data, index, dtype)
             dtype = None
@@ -500,8 +506,10 @@ def __init__(
         # create/copy the manager
         if isinstance(data, SingleBlockManager):
             if dtype is not None:
+                if not astype_is_view(data.dtype, pandas_dtype(dtype)):
+                    copy = False
                 data = data.astype(dtype=dtype)
-            elif copy:
+            if copy:
                 data = data.copy(deep=True)
         else:
             data = sanitize_array(data, index, dtype, copy)

pandas/tests/copy_view/test_constructors.py

@@ -47,14 +47,24 @@ def test_series_from_series(dtype):
     ser.iloc[0] = 0
     assert result.iloc[0] == 1
 
+    # forcing copy=False still gives a CoW shallow copy
+    result = Series(ser, dtype=dtype, copy=False)
+    assert np.shares_memory(get_array(ser), get_array(result))
+    assert result._mgr.blocks[0].refs.has_reference()
+
+    # forcing copy=True still results in an actual hard copy up front
+    result = Series(ser, dtype=dtype, copy=True)
+    assert not np.shares_memory(get_array(ser), get_array(result))
+    assert ser._mgr._has_no_reference(0)
+
 
 def test_series_from_series_with_reindex():
     # Case: constructing a Series from another Series with specifying an index
     # that potentially requires a reindex of the values
     ser = Series([1, 2, 3], name="name")
 
     # passing an index that doesn't actually require a reindex of the values
-    # -> without CoW we get an actual mutating view
+    # -> still getting a CoW shallow copy
     for index in [
         ser.index,
         ser.index.copy(),
@@ -66,6 +76,11 @@ def test_series_from_series_with_reindex():
         result.iloc[0] = 0
         assert ser.iloc[0] == 1
 
+        # forcing copy=True still results in an actual hard copy up front
+        result = Series(ser, index=index, copy=True)
+        assert not np.shares_memory(ser.values, result.values)
+        assert not result._mgr.blocks[0].refs.has_reference()
+
     # ensure that if an actual reindex is needed, we don't have any refs
     # (mutating the result wouldn't trigger CoW)
     result = Series(ser, index=[0, 1, 2, 3])
@@ -87,6 +102,13 @@ def test_series_from_array(idx, dtype, arr):
     arr[0] = 100
     tm.assert_series_equal(ser, ser_orig)
 
+    # if the user explicitly passes copy=False, we get an actual view
+    # not protected by CoW
+    ser = Series(arr, dtype=dtype, index=idx, copy=False)
+    assert np.shares_memory(get_array(ser), data)
+    arr[0] = 50
+    assert ser.iloc[0] == 50
+
 
 @pytest.mark.parametrize("copy", [True, False, None])
 def test_series_from_array_different_dtype(copy):
@@ -112,9 +134,22 @@ def test_series_from_index(idx):
     ser.iloc[0] = ser.iloc[1]
     tm.assert_index_equal(idx, expected)
 
+    # forcing copy=False still gives a CoW shallow copy
+    ser = Series(idx, copy=False)
+    assert np.shares_memory(get_array(ser), get_array(idx))
+    assert not ser._mgr._has_no_reference(0)
+    ser.iloc[0] = ser.iloc[1]
+    tm.assert_index_equal(idx, expected)
+
+    # forcing copy=True still results in a copy
+    ser = Series(idx, copy=True)
+    assert not np.shares_memory(get_array(ser), get_array(idx))
+    assert ser._mgr._has_no_reference(0)
 
-def test_series_from_index_different_dtypes():
-    idx = Index([1, 2, 3], dtype="int64")
+
+@pytest.mark.parametrize("copy", [True, False, None])
+def test_series_from_index_different_dtypes(copy):
+    idx = Index([1, 2, 3], dtype="int64", copy=copy)
     ser = Series(idx, dtype="int32")
     assert not np.shares_memory(get_array(ser), get_array(idx))
     assert ser._mgr._has_no_reference(0)