bpo-40703: Let PyType_FromSpec() set "type.__module__" only if it is not set yet. (GH-20273) (GH-20782)

(cherry picked from commit 24b8bad)

Author: miss-islington

Misc/NEWS.d/next/C API/2020-05-20-19-11-12.bpo-40703.qQXfW8.rst

@@ -0,0 +1,2 @@
+The PyType_FromSpec*() functions no longer overwrite the type's "__module__" attribute
+if it is set via "Py_tp_members" or "Py_tp_getset".

Objects/typeobject.c

@@ -3061,23 +3061,28 @@ PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
  }
 
  /* Set type.__module__ */
- s = strrchr(spec->name, '.');
- if (s != NULL) {
- int err;
- modname = PyUnicode_FromStringAndSize(
- spec->name, (Py_ssize_t)(s - spec->name));
- if (modname == NULL) {
+ if (_PyDict_GetItemIdWithError(type->tp_dict, &PyId___module__) == NULL) {
+ if (PyErr_Occurred()) {
  goto fail;
  }
- err = _PyDict_SetItemId(type->tp_dict, &PyId___module__, modname);
- Py_DECREF(modname);
- if (err != 0)
- goto fail;
- } else {
- if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
- "builtin type %.200s has no __module__ attribute",
- spec->name))
- goto fail;
+ s = strrchr(spec->name, '.');
+ if (s != NULL) {
+ int err;
+ modname = PyUnicode_FromStringAndSize(
+ spec->name, (Py_ssize_t)(s - spec->name));
+ if (modname == NULL) {
+ goto fail;
+ }
+ err = _PyDict_SetItemId(type->tp_dict, &PyId___module__, modname);
+ Py_DECREF(modname);
+ if (err != 0)
+ goto fail;
+ } else {
+ if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
+ "builtin type %.200s has no __module__ attribute",
+ spec->name))
+ goto fail;
+ }
  }
 
  return (PyObject*)res;