from dataclasses import dataclass, astuple from typing import DefaultDict, List from collections import defaultdict @dataclass class C: mp: DefaultDict[str, List] dd = defaultdict(list) dd["x"].append(12) c = C(mp=dd) d = astuple(c) # throws "TypeError: first argument must be callable or None" assert d == ({"x": [12]},) assert d[0] is not c.mp # make sure defaultdict is copied