[New-bugs-announce] [issue35540] dataclasses.asdict breaks with defaultdict fields

Will T report at bugs.python.org
Wed Dec 19 17:06:44 EST 2018

New submission from Will T <timwilloney at gmail.com>:

_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

    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.

components: Library (Lib)
messages: 332166
nosy: wrmsr
priority: normal
severity: normal
status: open
title: dataclasses.asdict breaks with defaultdict fields
versions: Python 3.7

Python tracker <report at bugs.python.org>

More information about the New-bugs-announce mailing list