Python how to create a dict of dict of list with defaultdict

Question

How do I create a dict of dict of lists using defaultdict? I am getting the following error.

>>> from collections import defaultdict
>>> a=defaultdict()
>>> a["testkey"]=None
>>> a
defaultdict(None, {'testkey': None})
>>> a["testkey"]["list"]=[]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object does not support item assignment

Answer 1

It's a little tricky. You make a defaultdict of defaultdicts, like so:

defaultdict(lambda: defaultdict(list))