py/objtype: Add type.__bases__ attribute.

Enabled as part of MICROPY_CPYTHON_COMPAT.

py/objtype.c

@@ -1014,6 +1014,21 @@ STATIC void type_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
             dest[0] = MP_OBJ_NEW_QSTR(self->name);
             return;
         }
+        if (attr == MP_QSTR___bases__) {
+            if (self == &mp_type_object) {
+                dest[0] = mp_const_empty_tuple;
+                return;
+            }
+            mp_obj_t parent_obj = self->parent ? MP_OBJ_FROM_PTR(self->parent) : MP_OBJ_FROM_PTR(&mp_type_object);
+            #if MICROPY_MULTIPLE_INHERITANCE
+            if (mp_obj_is_type(parent_obj, &mp_type_tuple)) {
+                dest[0] = parent_obj;
+                return;
+            }
+            #endif
+            dest[0] = mp_obj_new_tuple(1, &parent_obj);
+            return;
+        }
         #endif
         struct class_lookup_data lookup = {
             .obj = (mp_obj_instance_t*)self,

tests/basics/class_bases.py

@@ -0,0 +1,45 @@
+# test for type.__bases__ implementation
+
+if not hasattr(object, '__bases__'):
+    print("SKIP")
+    raise SystemExit
+
+class A:
+    pass
+
+class B(object):
+    pass
+
+class C(B):
+    pass
+
+class D(C, A):
+    pass
+
+# Check the attribute exists
+print(hasattr(A, '__bases__'))
+print(hasattr(B, '__bases__'))
+print(hasattr(C, '__bases__'))
+print(hasattr(D, '__bases__'))
+
+# Check it is always a tuple
+print(type(A.__bases__) == tuple)
+print(type(B.__bases__) == tuple)
+print(type(C.__bases__) == tuple)
+print(type(D.__bases__) == tuple)
+
+# Check size
+print(len(A.__bases__) == 1)
+print(len(B.__bases__) == 1)
+print(len(C.__bases__) == 1)
+print(len(D.__bases__) == 2)
+
+# Check values
+print(A.__bases__[0] == object)
+print(B.__bases__[0] == object)
+print(C.__bases__[0] == B)
+print(D.__bases__[0] == C)
+print(D.__bases__[1] == A)
+
+# Object has an empty tuple
+print(object.__bases__ == tuple())