BUG: Fix astype str issue 54654 (#54687)

* Fix issue #54654 on pickle roundtrip astype(str) might change original array even when copy is True * changelog * Update v2.2.0.rst rephrase * rephrase * Update lib.pyx add gh comment * Update v2.2.0.rst * Update lib.pyx fix CR --------- Co-authored-by: Itay Azolay <[email protected]>
pandas-dev · Oct 29, 2023 · a39f783 · a39f783
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diff --git a/doc/source/whatsnew/v2.2.0.rst b/doc/source/whatsnew/v2.2.0.rst
@@ -349,6 +349,7 @@ Numeric
 
 Conversion
 ^^^^^^^^^^
+- Bug in :func:`astype` when called with ``str`` on unpickled array - the array might change in-place (:issue:`54654`)
 - Bug in :meth:`Series.convert_dtypes` not converting all NA column to ``null[pyarrow]`` (:issue:`55346`)
 -
 

diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
@@ -792,7 +792,8 @@ cpdef ndarray[object] ensure_string_array(
 
     result = np.asarray(arr, dtype="object")
 
-    if copy and result is arr:
+    if copy and (result is arr or np.shares_memory(arr, result)):
+        # GH#54654
         result = result.copy()
     elif not copy and result is arr:
         already_copied = False

diff --git a/pandas/tests/copy_view/test_astype.py b/pandas/tests/copy_view/test_astype.py
@@ -1,3 +1,5 @@
+import pickle
+
 import numpy as np
 import pytest
 
@@ -130,6 +132,15 @@ def test_astype_string_and_object_update_original(
     tm.assert_frame_equal(df2, df_orig)
 
 
+def test_astype_string_copy_on_pickle_roundrip():
+    # https://github.com/pandas-dev/pandas/issues/54654
+    # ensure_string_array may alter array inplace
+    base = Series(np.array([(1, 2), None, 1], dtype="object"))
+    base_copy = pickle.loads(pickle.dumps(base))
+    base_copy.astype(str)
+    tm.assert_series_equal(base, base_copy)
+
+
 def test_astype_dict_dtypes(using_copy_on_write):
     df = DataFrame(
         {"a": [1, 2, 3], "b": [4, 5, 6], "c": Series([1.5, 1.5, 1.5], dtype="float64")}