More Pythonic way of counting things in a heavily nested defaultdict

Question

My code currently has to count things in a heavily nested dict into another. I have items that need to be indexed by 3 values and then counted. So, before my loop, I initialize a nested defaultdict like so:

from collections import defaultdict

type_to_count_dic = defaultdict(
        lambda: defaultdict(
            lambda: defaultdict(int)
        )
    )

Which allows me to count the items within a tight loop like so:

for a in ...:
    for b in ...:
        for c in ...:
            type_to_count_dic[a][b][c] += 1

I feel like initializing all those defaultdicts feels a lot like making a type declaration in something like Java. Is there a more idiomatic/Pythonic way of doing something like this?

Answer 1

from collections import defaultdict

class _defaultdict(defaultdict):
    def __add__(self, other):
        return other

def CountTree():
    return _defaultdict(CountTree)

---

>>> t = CountTree()
>>> t['a']
defaultdict(<function CountTree at 0x9e5c3ac>, {})
>>> t['a']['b']['c'] += 1
>>> print t['a']['b']['c']
1