Populating a defaultdict at init time

Question

How can I get a callable factory for a defaultdict to allow populating it with a comprehension? I think it's probably not possible, but I can't think of a good reason why?

>>> def foo(*args):
...     # TODO
...
>>> from collections import defaultdict
>>> thing = foo(defaultdict, int)
>>> d = thing((i, i*i) for i in range(3))
>>> d[2]
# should return 4
>>> d[-1]
# should return 0

Answer 1

Any arguments to defaultdict after the default_factory are treated just like arguments to dict:

>>> defaultdict(int, [(i, i*i) for i in range(5)])
defaultdict(<type 'int'>, {0: 0, 1: 1, 2: 4, 3: 9, 4: 16})

Just pass the comprehension to defaultdict and let it do the work:

def defaultdict_factory_factory(default_factory):
    def defaultdict_factory(*args, **kwargs):
        return defaultdict(default_factory, *args, **kwargs)
    return defaultdict_factory

Or use functools.partial:

def defaultdict_factory_factory(default_factory):
    return partial(defaultdict, default_factory)