defaultloco.py

class mydefaultdict(dict):
    def __init__(self, default, **kwargs):
        super(mydefaultdict, self).__init__(**kwargs)
        self.default = default
        
    def __getitem__(self, item):
        try:
            return super(mydefaultdict, self).__getitem__(item)
        except KeyError:
            val = self.default(item)
            self[item] = val
            return val

https://gist.github.com/msemelman/7fd64ec2e3eb3781df69