python nested dictionary

from collections import defaultdict

class my_nested_dict(defaultdict, dict):
    def __init__(self):
        defaultdict.__init__(self, my_nested_dict)

    def __repr__(self):
        return dict.__repr__(self)

###
###        pd = my_nested_dict()
###        pd['dsrc_code'] = 'NATIONAL_IDENTITY'
###        pd['dsrc_ACCT'] = 'p1'
###        pd['tim']['was']['here'] = 42
###        pprint(pd)