BUG: astype to string modifying input array inplace

doc/source/whatsnew/v2.0.0.rst

@@ -1023,6 +1023,7 @@ Conversion
 - Bug where any :class:`ExtensionDtype` subclass with ``kind="M"`` would be interpreted as a timezone type (:issue:`34986`)
 - Bug in :class:`.arrays.ArrowExtensionArray` that would raise ``NotImplementedError`` when passed a sequence of strings or binary (:issue:`49172`)
 - Bug in :meth:`Series.astype` raising ``pyarrow.ArrowInvalid`` when converting from a non-pyarrow string dtype to a pyarrow numeric type (:issue:`50430`)
+- Bug in :meth:`DataFrame.astype` modifying input array inplace when converting to ``string`` and ``copy=False`` (:issue:`51073`)
 - Bug in :meth:`Series.to_numpy` converting to NumPy array before applying ``na_value`` (:issue:`48951`)
 - Bug in :meth:`DataFrame.astype` not copying data when converting to pyarrow dtype (:issue:`50984`)
 - Bug in :func:`to_datetime` was not respecting ``exact`` argument when ``format`` was an ISO8601 format (:issue:`12649`)

pandas/_libs/lib.pyx

@@ -739,6 +739,7 @@ cpdef ndarray[object] ensure_string_array(
     """
     cdef:
         Py_ssize_t i = 0, n = len(arr)
+        bint already_copied = True
 
     if hasattr(arr, "to_numpy"):
 
@@ -757,6 +758,8 @@ cpdef ndarray[object] ensure_string_array(
 
     if copy and result is arr:
         result = result.copy()
+    elif not copy and result is arr:
+        already_copied = False
 
     if issubclass(arr.dtype.type, np.str_):
         # short-circuit, all elements are str
@@ -768,6 +771,10 @@ cpdef ndarray[object] ensure_string_array(
         if isinstance(val, str):
             continue
 
+        elif not already_copied:
+            result = result.copy()
+            already_copied = True
+
         if not checknull(val):
             if not util.is_float_object(val):
                 # f"{val}" is faster than str(val)

pandas/tests/arrays/string_/test_string.py

@@ -290,13 +290,9 @@ def test_constructor_nan_like(na):
 
 @pytest.mark.parametrize("copy", [True, False])
 def test_from_sequence_no_mutate(copy, cls, request):
-    if cls is ArrowStringArray and copy is False:
-        mark = pytest.mark.xfail(
-            raises=AssertionError, reason="numpy array are different"
-        )
-        request.node.add_marker(mark)
 
     nan_arr = np.array(["a", np.nan], dtype=object)
+    expected_input = nan_arr.copy()
     na_arr = np.array(["a", pd.NA], dtype=object)
 
     result = cls._from_sequence(nan_arr, copy=copy)
@@ -309,9 +305,7 @@ def test_from_sequence_no_mutate(copy, cls, request):
         expected = cls(na_arr)
 
     tm.assert_extension_array_equal(result, expected)
-
-    expected = nan_arr if copy else na_arr
-    tm.assert_numpy_array_equal(nan_arr, expected)
+    tm.assert_numpy_array_equal(nan_arr, expected_input)
 
 
 def test_astype_int(dtype):

pandas/tests/frame/methods/test_astype.py

@@ -879,3 +879,13 @@ def test_astype_copies(dtype):
     df.iloc[0, 0] = 100
     expected = DataFrame({"a": [1, 2, 3]}, dtype="int64[pyarrow]")
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize("val", [None, 1, 1.5, np.nan, NaT])
+def test_astype_to_string_not_modifying_input(string_storage, val):
+    # GH#51073
+    df = DataFrame({"a": ["a", "b", val]})
+    expected = df.copy()
+    with option_context("mode.string_storage", string_storage):
+        df.astype("string", copy=False)
+    tm.assert_frame_equal(df, expected)