Thursday, February 4, 2010


collections.defaultdict is nice, especially when counting things. But, defaultdict only lets you use zero-argument constructors. Pffft! Fortunately, it's easy to write a defaultdict which passes arguments to the constructor:

class defaultdict2(dict):
    def __init__(self, factory, factArgs=(), dictArgs=()):
        dict.__init__(self, *dictArgs)
        self.factory = factory
        self.factArgs = factArgs
    def __missing__(self, key):
        self[key] = self.factory(*self.factArgs)
        return self[key]

Update 2/8/10: added "return" line to __missing__ per discussion in this post on __missing__.

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