gh-160427: Return None from TextIOWrapper.buffer when detached (GH-160802)

gh-154523 turned the buffer member descriptor into a getter that raises
ValueError when the buffer is NULL, and the backport shipped in 3.14.9
and 3.15.1. That made hasattr(stream, "buffer") raise after detach().
Restore the old member descriptor result (None) while keeping the
critical section.
(cherry picked from commit 3422197)

Co-authored-by: Cody Maloney <cmaloney@users.noreply.github.com>

Author: cmaloney

Lib/test/test_io/test_textio.py

@@ -243,6 +243,7 @@ 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")
@@ -258,6 +259,7 @@ 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"))
@@ -1713,9 +1715,7 @@ 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)
 
 
 class PyTextIOWrapperTest(TextIOWrapperTest, PyTestCase):

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

@@ -3453,7 +3453,11 @@ 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[] = {