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pythontypesmypy
Anthony Naddeo
Anthony Naddeo

Reputation: 2751

mypy types and optional filtering

New to mypy and I'm having trouble finding the right way to get types working when filtering optionals from collections.

from typing import Optional, List

list1: List[Optional[int]] = [1, None, 3]
# This type is an error, filter can't even handle a list with optionals
list2: List[int] = list(filter(lambda n: n is not None, list1))

Should I be casting this? Should mypy be able to infer that None is filtered out now?

Upvotes: 6

Views: 2429

Answers (1)

juanpa.arrivillaga
juanpa.arrivillaga

Reputation: 96181

Using typing.cast should work, but I think the best solution here is to use a list comprehension.

The following code passes mypy for me:

from typing import Optional, List

list1: List[Optional[int]] = [1, None, 3]
list2: List[int] = [x for x in list1 if x is not None]

And in general, list comprehensions are the more idiomatic way of expressing mapping/filtering operations. (Guido actually wanted to remove map/filter in Python 3!)

Upvotes: 8

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