dc.py

import dataclasses
import collections
import typing

@dataclasses.dataclass
class Foo:
    d: typing.DefaultDict[str, int] = dataclasses.field(
            default_factory=lambda: collections.defaultdict(int))

f = Foo()
print(f.d["asdf"])
print(dataclasses.asdict(f))

output

Traceback (most recent call last):
  File "dc.py", line 12, in <module>
    print(dataclasses.asdict(f))
  File "/usr/lib/python3.7/dataclasses.py", line 1044, in asdict
    return _asdict_inner(obj, dict_factory)
  File "/usr/lib/python3.7/dataclasses.py", line 1051, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
  File "/usr/lib/python3.7/dataclasses.py", line 1083, in _asdict_inner
    for k, v in obj.items())
TypeError: first argument must be callable or None