python equivalent of functools 'partial' for a class / constructor

Question

I want to create a class that behaves like collections.defaultdict, without having the usage code specify the factory. EG: instead of

class Config(collections.defaultdict):
    pass

this:

Config = functools.partial(collections.defaultdict, list)

This almost works, but

isinstance(Config(), Config)

fails. I am betting this clue means there are more devious problems deeper in also. So is there a way to actually achieve this?

I also tried:

class Config(Object):
    __init__ = functools.partial(collections.defaultdict, list)

Answer 1

I don't think there's a standard method to do it, but if you need it often, you can just put together your own small function:

import functools
import collections


def partialclass(cls, *args, **kwds):

    class NewCls(cls):
        __init__ = functools.partialmethod(cls.__init__, *args, **kwds)

    return NewCls


if __name__ == '__main__':
    Config = partialclass(collections.defaultdict, list)
    assert isinstance(Config(), Config)