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

Lib/test/test_io/test_textio.py

@@ -260,6 +260,10 @@ def test_detach(self):
         self.assertFalse(t.line_buffering)
         self.assertFalse(t.write_through)
 
+        # The attribute is None once detached, not an error (gh-160427).
+        self.assertIsNone(t.buffer)
+        self.assertTrue(hasattr(t, "buffer"))
+
     def test_repr(self):
         raw = self.BytesIO("hello".encode("utf-8"))
         b = self.BufferedReader(raw)
@@ -1710,9 +1714,9 @@ 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')
+                # gh-160427: once detached the attribute is None, not an
+                # error; the read above still had to complete safely.
+                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/2026-10-23-19-02-11.gh-issue-160427.k3x9aq.rst

@@ -0,0 +1,4 @@
+Fix ``io.TextIOWrapper.buffer`` to return ``None`` after the wrapper has been
+detached, instead of raising :exc:`ValueError`.  This restores the behaviour of
+Python 3.14.8 and earlier, and stops :func:`hasattr` from raising when called on
+a detached wrapper.

Modules/_io/textio.c

@@ -3467,7 +3467,19 @@ static PyObject *
 _io_TextIOWrapper_buffer_get_impl(textio *self)
 /*[clinic end generated code: output=d265a34555aa5d4b input=5951cfa148f7350a]*/
 {
-    return buffer_access_safe(self);
+    if (self->buffer == NULL) {
+        if (self->ok <= 0) {
+            PyErr_SetString(PyExc_ValueError,
+                            "I/O operation on uninitialized object");
+            return NULL;
+        }
+        /* Detached: the attribute is None, as it was while it was a member
+           descriptor (gh-160427). Raising here broke hasattr() in 3.14.9. */
+        Py_RETURN_NONE;
+    }
+
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
+    return Py_NewRef(self->buffer);
 }
 
 static PyMethodDef incrementalnewlinedecoder_methods[] = {