Type hints for nested defaultdict

Question

What is the right way to write type hints for defaultdict(lambda: defaultdict(set))?

I use Python 3.10.5 and mypy 0.971, and find mypy returns an error because var = defaultdict(lambda: defaultdict(set)) doesn’t have a type hint.

Premises

All keys of the first defaultdict and the second defaultdict are str.
Values of the first defaultdict are defaultdict. Values of the second defaultdict are set. (This may be obvious.)

Sample code

from collections import defaultdict
var = defaultdict(lambda: defaultdict(set))

Output

test.py:2: error: Need type annotation for "var"

Asked By: dmjy

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Source

Answer 1

Starting from Python 3.9 you can use defaultdict and set itself as annotations:

var: defaultdict[str, defaultdict[str, set]] = defaultdict(lambda: defaultdict(set))

For earlier Python versions there’s a special DefaultDict type from typing module:

from collections import defaultdict
from typing import DefaultDict, Set

var: DefaultDict[str, DefaultDict[str, Set]] = defaultdict(lambda: defaultdict(set))

Answered By: funnydman

Type hints for nested defaultdict

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