gh-160427: Return None from TextIOWrapper.buffer when detached

Lib/test/test_io/test_textio.py

@@ -244,6 +244,7 @@ def test_detach(self):
         r = self.BytesIO()
         b = self.BufferedWriter(r)
         t = self.TextIOWrapper(b, encoding="ascii")
+        self.assertIs(t.buffer, b)
         self.assertIs(t.detach(), b)
 
         t = self.TextIOWrapper(b, encoding="ascii")
@@ -259,6 +260,7 @@ def test_detach(self):
         self.assertEqual(t.errors, "strict")
         self.assertFalse(t.line_buffering)
         self.assertFalse(t.write_through)
+        self.assertIsNone(t.buffer)
 
     def test_repr(self):
         raw = self.BytesIO("hello".encode("utf-8"))
@@ -1721,9 +1723,7 @@ def readinto(self, b):
                     self.BufferedReader(raw), encoding="utf-8")
                 method = getattr(wrapper, method_name)
                 self.assertEqual(method(), "ab\n")
-                with self.assertRaisesRegex(ValueError,
-                                            "underlying buffer has been detached"):
-                    getattr(wrapper, 'buffer')
+                self.assertIsNone(wrapper.buffer)
 
     def test_reentrant_seek_during_tell(self):
         # gh-153539: reading short of _CHUNK_SIZE leaves residual bytes in the

Misc/NEWS.d/next/Library/2026-10-26-16-15-29.gh-issue-160427.9G4EBb.rst

@@ -0,0 +1,2 @@
+Fix :attr:`~io.TextIOBase.buffer` of a detached :class:`io.TextIOWrapper`
+raising :exc:`ValueError` instead of returning ``None``.

Modules/_io/textio.c

@@ -3474,7 +3474,11 @@ static PyObject *
 _io_TextIOWrapper_buffer_get_impl(textio *self)
 /*[clinic end generated code: output=d265a34555aa5d4b input=5951cfa148f7350a]*/
 {
-    return buffer_access_safe(self);
+    /* gh-160427: None once detached, matching the former member descriptor. */
+    if (self->buffer == NULL) {
+        Py_RETURN_NONE;
+    }
+    return Py_NewRef(self->buffer);
 }
 
 static PyMethodDef incrementalnewlinedecoder_methods[] = {