Backport PR pandas-dev#52017: BUG: Series constructor not respecting CoW when called with BlockManager

Author: phofl

pandas/core/series.py

@@ -379,6 +379,8 @@ def __init__(
             and dtype is None
             and copy is False
         ):
+            if using_copy_on_write():
+                data = data.copy(deep=False)
             # GH#33357 called with just the SingleBlockManager
             NDFrame.__init__(self, data)
             if fastpath:
@@ -397,13 +399,18 @@ def __init__(
                     data = SingleBlockManager.from_array(data, index)
                 elif manager == "array":
                     data = SingleArrayManager.from_array(data, index)
+            elif using_copy_on_write() and not copy:
+                data = data.copy(deep=False)
             if copy:
                 data = data.copy()
             # skips validation of the name
             object.__setattr__(self, "_name", name)
             NDFrame.__init__(self, data)
             return
 
+        if isinstance(data, SingleBlockManager) and using_copy_on_write() and not copy:
+            data = data.copy(deep=False)
+
         name = ibase.maybe_extract_name(name, data, type(self))
 
         if index is not None:

pandas/tests/copy_view/test_constructors.py

@@ -1,6 +1,7 @@
 import numpy as np
 import pytest
 
+import pandas as pd
 from pandas import (
     DataFrame,
     DatetimeIndex,
@@ -118,6 +119,33 @@ def test_series_from_index_different_dtypes(using_copy_on_write):
         assert ser._mgr._has_no_reference(0)
 
 
+@pytest.mark.parametrize("fastpath", [False, True])
+@pytest.mark.parametrize("dtype", [None, "int64"])
+@pytest.mark.parametrize("idx", [None, pd.RangeIndex(start=0, stop=3, step=1)])
+def test_series_from_block_manager(using_copy_on_write, idx, dtype, fastpath):
+    ser = Series([1, 2, 3], dtype="int64")
+    ser_orig = ser.copy()
+    ser2 = Series(ser._mgr, dtype=dtype, fastpath=fastpath, index=idx)
+    assert np.shares_memory(get_array(ser), get_array(ser2))
+    if using_copy_on_write:
+        assert not ser2._mgr._has_no_reference(0)
+
+    ser2.iloc[0] = 100
+    if using_copy_on_write:
+        tm.assert_series_equal(ser, ser_orig)
+    else:
+        expected = Series([100, 2, 3])
+        tm.assert_series_equal(ser, expected)
+
+
+def test_series_from_block_manager_different_dtype(using_copy_on_write):
+    ser = Series([1, 2, 3], dtype="int64")
+    ser2 = Series(ser._mgr, dtype="int32")
+    assert not np.shares_memory(get_array(ser), get_array(ser2))
+    if using_copy_on_write:
+        assert ser2._mgr._has_no_reference(0)
+
+
 @pytest.mark.parametrize("func", [lambda x: x, lambda x: x._mgr])
 @pytest.mark.parametrize("columns", [None, ["a"]])
 def test_dataframe_constructor_mgr_or_df(using_copy_on_write, columns, func):