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pythonlambdacollectionsdefaultdict

Python defaultdict

I found something strange that I couldn't understand. This is the case:

from collections import defaultdict
a = defaultdict(lambda: len(a))

This is just the part of the code, and the code has never defined 'a' above.

The questions are:

  • Is it possible to use defaultdict as is, not specifying the variable previously?
  • If possible, what is the meaning of that code?
Solution

  • Maybe it is best explained in an example:

    >>> a = defaultdict(lambda: len(b))
>>> b = 'abcd'
>>> a[0]
4

    As you can see, it is possible to use b in the lambda even though the b does not yet exist at that point. What is important is that b exists at the time when the lambda is executed. At that point, Python will look for a variable named b and use it.

    Note also that the original code does not necessarily use length of the defaultdict itself. It simply evaluates whatever a is at that point. See this example:

    >>> a = defaultdict(lambda: len(a))
>>> a['a']
0
>>> a['b']
1

    So far, so good. But then rename some things:

    >>> x = a
>>> a = []
>>> x['c']
0
>>> x['d']
0

    Now the deaultdict is named x, but it does not use len(x). It still uses len(a). This caveat may become important if you sent the defaultdict to a function where a does not mean anything.