How to convert defaultdict of defaultdicts [of defaultdicts] to dict of dicts [of dicts]?

You can recurse over the tree, replacing each defaultdict instance with a dict produced by a dict comprehension:

def default_to_regular(d):
    if isinstance(d, defaultdict):
        d = {k: default_to_regular(v) for k, v in d.items()}
    return d

Demo:

>>> from collections import defaultdict
>>> factory = lambda: defaultdict(factory)
>>> defdict = factory()
>>> defdict['one']['two']['three']['four'] = 5
>>> defdict
defaultdict(<function <lambda> at 0x103098ed8>, {'one': defaultdict(<function <lambda> at 0x103098ed8>, {'two': defaultdict(<function <lambda> at 0x103098ed8>, {'three': defaultdict(<function <lambda> at 0x103098ed8>, {'four': 5})})})})
>>> default_to_regular(defdict)
{'one': {'two': {'three': {'four': 5}}}}