Nested defaultdict of defaultdict

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The other answers here tell you how to create a defaultdict which contains “infinitely many” defaultdict, but they fail to address what I think may have been your initial need which was to simply have a two-depth defaultdict.

You may have been looking for: 

defaultdict(lambda: defaultdict(dict))

The reasons why you might prefer this construct are:

  • It is more explicit than the recursive solution, and therefore likely more understandable to the reader.
  • This enables the “leaf” of the defaultdict to be something other than a dictionary, e.g.,: defaultdict(lambda: defaultdict(list)) or defaultdict(lambda: defaultdict(set))

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