How Can I Create an Infinite-Level Recursive defaultdict in Python?

Creating an Infinite-Level Recursive defaultdict

In Python, defaultdict is a versatile tool that provides a default value for missing keys in dictionaries. However, is it possible to create an infinite-level recursive defaultdict? This means creating a defaultdict where the default value is also a defaultdict, effectively creating nested defaultdicts.

Initially, one might expect that x = defaultdict(defaultdict) would create a two-level defaultdict, but accessing x[0][0] results in a KeyError. To achieve infinite-level recursion, an alternative approach is needed.

One solution is to use a lambda function as the default value:

x = defaultdict(lambda: defaultdict(dict))

This creates a defaultdict where the default value is a function that returns another defaultdict with a dictionary as its default value. This allows for infinite-level recursion, as accessing x[0][1][0] creates an empty dictionary within the nested defaultdicts.

Compared to the recursive method presented in other answers, this approach offers several advantages:

• Explicitness: It is more explicit about the nested structure, making it easier to understand.
• Flexibility: It allows the "leaf" of the defaultdict structure to be something other than a dictionary. For example, defaultdict(lambda: defaultdict(list)) would create a defaultdict with lists as leaf values.

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