BUG (string dtype): fix inplace mutation with copy=False in ensure_string_array (pandas-dev#59756)

* BUG (string dtype): fix inplace mutation with copy=False in ensure_string_array

* update

Author: jorisvandenbossche

pandas/_libs/lib.pyx

@@ -733,7 +733,9 @@ cpdef ndarray[object] ensure_string_array(
     convert_na_value : bool, default True
         If False, existing na values will be used unchanged in the new array.
     copy : bool, default True
-        Whether to ensure that a new array is returned.
+        Whether to ensure that a new array is returned. When True, a new array
+        is always returned. When False, a new array is only returned when needed
+        to avoid mutating the input array.
     skipna : bool, default True
         Whether or not to coerce nulls to their stringified form
         (e.g. if False, NaN becomes 'nan').
@@ -762,11 +764,15 @@ cpdef ndarray[object] ensure_string_array(
 
     result = np.asarray(arr, dtype="object")
 
-    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
+    if result is arr or np.may_share_memory(arr, result):
+        # if np.asarray(..) did not make a copy of the input arr, we still need
+        #  to do that to avoid mutating the input array
+        # GH#54654: share_memory check is needed for rare cases where np.asarray
+        #  returns a new object without making a copy of the actual data
+        if copy:
+            result = result.copy()
+        else:
+            already_copied = False
     elif not copy and not result.flags.writeable:
         # Weird edge case where result is a view
         already_copied = False

pandas/tests/copy_view/test_astype.py

@@ -7,7 +7,6 @@
 
 from pandas.compat import HAS_PYARROW
 from pandas.compat.pyarrow import pa_version_under12p0
-import pandas.util._test_decorators as td
 
 from pandas import (
     DataFrame,
@@ -111,7 +110,8 @@ def test_astype_string_and_object_update_original(dtype, new_dtype):
     tm.assert_frame_equal(df2, df_orig)
 
 
-def test_astype_string_copy_on_pickle_roundrip():
+def test_astype_str_copy_on_pickle_roundrip():
+    # TODO(infer_string) this test can be removed after 3.0 (once str is the default)
     # 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"))
@@ -120,14 +120,22 @@ def test_astype_string_copy_on_pickle_roundrip():
     tm.assert_series_equal(base, base_copy)
 
 
-@td.skip_if_no("pyarrow")
-def test_astype_string_read_only_on_pickle_roundrip():
+def test_astype_string_copy_on_pickle_roundrip(any_string_dtype):
+    # 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(any_string_dtype)
+    tm.assert_series_equal(base, base_copy)
+
+
+def test_astype_string_read_only_on_pickle_roundrip(any_string_dtype):
     # https://github.com/pandas-dev/pandas/issues/54654
     # ensure_string_array may alter read-only array inplace
     base = Series(np.array([(1, 2), None, 1], dtype="object"))
     base_copy = pickle.loads(pickle.dumps(base))
     base_copy._values.flags.writeable = False
-    base_copy.astype("string[pyarrow]")
+    base_copy.astype(any_string_dtype)
     tm.assert_series_equal(base, base_copy)
 
 

pandas/tests/libs/test_lib.py

@@ -1,3 +1,5 @@
+import pickle
+
 import numpy as np
 import pytest
 
@@ -283,3 +285,15 @@ def test_no_default_pickle():
     # GH#40397
     obj = tm.round_trip_pickle(lib.no_default)
     assert obj is lib.no_default
+
+
+def test_ensure_string_array_copy():
+    # ensure the original array is not modified in case of copy=False with
+    # pickle-roundtripped object dtype array
+    # https://github.com/pandas-dev/pandas/issues/54654
+    arr = np.array(["a", None], dtype=object)
+    arr = pickle.loads(pickle.dumps(arr))
+    result = lib.ensure_string_array(arr, copy=False)
+    assert not np.shares_memory(arr, result)
+    assert arr[1] is None
+    assert result[1] is np.nan