BUG: ensure_string_array might modify read-only array inplace (pandas-dev#57212)

* BUG: ensure_string_array might modify read-only array inplace

* BUG: ensure_string_array might modify read-only array inplace

* Fix pyarrow installed error

Author: phofl

doc/source/whatsnew/v3.0.0.rst

@@ -159,7 +159,7 @@ Numeric
 
 Conversion
 ^^^^^^^^^^
--
+- Bug in :meth:`Series.astype` might modify read-only array inplace when casting to a string dtype (:issue:`57212`)
 -
 
 Strings

pandas/_libs/lib.pyx

@@ -770,6 +770,9 @@ cpdef ndarray[object] ensure_string_array(
         result = result.copy()
     elif not copy and result is arr:
         already_copied = False
+    elif not copy and not result.flags.writeable:
+        # Weird edge case where result is a view
+        already_copied = False
 
     if issubclass(arr.dtype.type, np.str_):
         # short-circuit, all elements are str

pandas/tests/copy_view/test_astype.py

@@ -4,6 +4,7 @@
 import pytest
 
 from pandas.compat.pyarrow import pa_version_under12p0
+import pandas.util._test_decorators as td
 
 import pandas as pd
 from pandas import (
@@ -139,6 +140,17 @@ 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():
+    # 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]", copy=False)
+    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")}