An extension of collections.defaultdict that I wish were in the stdlib.

factory_dict.py

import inspect
from types import FunctionType
from collections import defaultdict

class FactoryDict(defaultdict):
    """
    >>> fd = FactoryDict(int)
    >>> fd['foo'] += 7
    >>> fd['foo']
    7
    >>>
    >>> fd = FactoryDict(lambda x: x**2)
    >>> fd[3] += 7
    >>> fd[3]
    16
    """
    def __init__(self, default_factory):
        defaultdict.__init__(self, default_factory)
        self.factory_takes_args = isinstance(default_factory, FunctionType) and any(inspect.getargspec(default_factory))

    def __missing__(self, item):
        val = self.default_factory(item) if self.factory_takes_args else self.default_factory()
        self[item] = val
        return val