py: Don't try to "bind" types store as attributes of objects.

This was hit when trying to make urlparse.py from stdlib run. Took
quite some time to debug.

TODO: Reconsile bound method creation process better, maybe callable is
to generic type to bind at all?

py/objtype.c

@@ -333,6 +333,9 @@ STATIC void instance_convert_return_attr(mp_obj_t self, mp_obj_t member, mp_obj_
         // return a bound method, with self being the type of this object
         dest[0] = ((mp_obj_static_class_method_t*)member)->fun;
         dest[1] = mp_obj_get_type(self);
+    } else if (MP_OBJ_IS_TYPE(member, &mp_type_type)) {
+        // Don't try to bind types
+        dest[0] = member;
     } else if (mp_obj_is_callable(member)) {
         // return a bound method, with self being this object
         dest[0] = member;

py/runtime.c

@@ -840,6 +840,9 @@ void mp_load_method_maybe(mp_obj_t base, qstr attr, mp_obj_t *dest) {
                 // return a bound method, with self being the type of this object
                 dest[0] = ((mp_obj_static_class_method_t*)elem->value)->fun;
                 dest[1] = mp_obj_get_type(base);
+            } else if (MP_OBJ_IS_TYPE(elem->value, &mp_type_type)) {
+                // Don't try to bind types
+                dest[0] = elem->value;
             } else if (mp_obj_is_callable(elem->value)) {
                 // return a bound method, with self being this object
                 dest[0] = elem->value;

tests/basics/class_store_class.py

@@ -0,0 +1,44 @@
+# Inspired by urlparse.py from CPython 3.3 stdlib
+# There was a bug in MicroPython that under some conditions class stored
+# in instance attribute later was returned "bound" as if it was a method,
+# which caused class constructor to receive extra argument.
+from collections import namedtuple
+
+_DefragResultBase = namedtuple('DefragResult', 'foo bar')
+
+class _ResultMixinStr(object):
+    def encode(self):
+        return self._encoded_counterpart(*(x.encode() for x in self))
+
+class _ResultMixinBytes(object):
+    def decode(self):
+        return self._decoded_counterpart(*(x.decode() for x in self))
+
+class DefragResult(_DefragResultBase, _ResultMixinStr):
+    pass
+
+class DefragResultBytes(_DefragResultBase, _ResultMixinBytes):
+    pass
+
+
+DefragResult._encoded_counterpart = DefragResultBytes
+DefragResultBytes._decoded_counterpart = DefragResult
+
+# Due to differences in type and native subclass printing,
+# the best thing we can do here is to just test that no exceptions
+# happen
+
+#print(DefragResult, DefragResult._encoded_counterpart)
+#print(DefragResultBytes, DefragResultBytes._decoded_counterpart)
+
+o1 = DefragResult("a", "b")
+#print(o1, type(o1))
+o2 = DefragResultBytes("a", "b")
+#print(o2, type(o2))
+
+#print(o1._encoded_counterpart)
+_o1 = o1.encode()
+print(_o1[0], _o1[1])
+#print(_o1, type(_o1))
+
+print("All's ok")