collections.defaultdict’s “first argument must be callable”

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  • dictionary
  • defaultdict

A defaultdict won’t work if the desired default value is provided directly:

>>> from collections import defaultdict
>>> dct = defaultdict(float('-inf'))

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    dct = defaultdict(float('-inf'))
TypeError: first argument must be callable

Providing e.g. a lambda expression instead solves the problem:

>>> dct = defaultdict(lambda: float('-inf'))