Issue #26906: Resolving special methods of uninitialized type now causes

implicit initialization of the type instead of a fail.

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #26906: Resolving special methods of uninitialized type now causes
+  implicit initialization of the type instead of a fail.
+
 - Issue #18287: PyType_Ready() now checks that tp_name is not NULL.
   Original patch by Niklas Koep.
 

Objects/typeobject.c

@@ -2914,11 +2914,25 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
     /* Look in tp_dict of types in MRO */
     mro = type->tp_mro;
 
-    /* If mro is NULL, the type is either not yet initialized
-       by PyType_Ready(), or already cleared by type_clear().
-       Either way the safest thing to do is to return NULL. */
-    if (mro == NULL)
-        return NULL;
+    if (mro == NULL) {
+        if ((type->tp_flags & Py_TPFLAGS_READYING) == 0 &&
+            PyType_Ready(type) < 0) {
+            /* It's not ideal to clear the error condition,
+               but this function is documented as not setting
+               an exception, and I don't want to change that.
+               When PyType_Ready() can't proceed, it won't
+               set the "ready" flag, so future attempts to ready
+               the same type will call it again -- hopefully
+               in a context that propagates the exception out.
+            */
+            PyErr_Clear();
+            return NULL;
+        }
+        mro = type->tp_mro;
+        if (mro == NULL) {
+            return NULL;
+        }
+    }
 
     res = NULL;
     /* keep a strong reference to mro because type->tp_mro can be replaced