asdict breaks with defaultdicts

[seansfkelley]

https://bugs.python.org/issue35540 is the official bug report on the stdlib implementation in 3.7, copied below. There's a proposed fix at python/cpython#11361 to special-case defaultdict.

_asdict_inner attempts to manually recursively deepcopy dicts by calling type(obj) with a generator of transformed keyvalue tuples @ https://github.com/python/cpython/blob/b2f642ccd2f65d2f3bf77bbaa103dd2bc2733734/Lib/dataclasses.py#L1080 . defaultdicts are dicts so this runs but unlike other dicts their first arg has to be a callable or None:

import collections
import dataclasses as dc

@dc.dataclass()
class C:
    d: dict

c = C(collections.defaultdict(lambda: 3, {}))
d = dc.asdict(c)

assert isinstance(d['d'], collections.defaultdict)
assert d['d']['a'] == 3

=>

Traceback (most recent call last):
  File "boom.py", line 9, in <module>
    d = dc.asdict(c)
  File "/Users/spinlock/.pyenv/versions/3.7.1/lib/python3.7/dataclasses.py", line 1019, in asdict
    return _asdict_inner(obj, dict_factory)
  File "/Users/spinlock/.pyenv/versions/3.7.1/lib/python3.7/dataclasses.py", line 1026, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
  File "/Users/spinlock/.pyenv/versions/3.7.1/lib/python3.7/dataclasses.py", line 1058, in _asdict_inner
    for k, v in obj.items())
TypeError: first argument must be callable or None

I understand that it isn't this bit of code's job to support every dict (and list etc.) subclass under the sun but given defaultdict is stdlib it's imo worth supporting explicitly.