Fix issue pandas-dev#54654

on pickle roundtrip astype(str) might change original array even when copy is True

Author: itay-jether

pandas/_libs/lib.pyx

@@ -775,7 +775,7 @@ 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.may_share_memory(arr, result)):
         result = result.copy()
     elif not copy and result is arr:
         already_copied = False

pandas/tests/copy_view/test_astype.py

@@ -1,3 +1,5 @@
+import pickle
+
 import numpy as np
 import pytest
 
@@ -131,6 +133,15 @@ 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")}