py: Implement base class lookup, issubclass, isinstance.
diff --git a/py/objtuple.c b/py/objtuple.c
index 0050fc5..7685cc4 100644
--- a/py/objtuple.c
+++ b/py/objtuple.c
@@ -136,6 +136,7 @@
 }
 
 void mp_obj_tuple_get(mp_obj_t self_in, uint *len, mp_obj_t **items) {
+    assert(MP_OBJ_IS_TYPE(self_in, &tuple_type));
     mp_obj_tuple_t *self = self_in;
     *len = self->len;
     *items = &self->items[0];