CODEWITHSUNDEEP
pythonpython-typing
Jim Newton
Jim Newton

Reputation: 652

how to declare type of list comprehension in python

What is the correct way to declare the type of the following function in Python?

from typing import TypeVar, List

T = TypeVar('T')      # Declare type variable

def dual_combinator(self, a: List[T], b: List[T]) -> List[T]:
    return [x for x in a if x in b]

I'd like the type to indicate that the input variables a and b can be anything which is compatible with a list comprehension, and the in operator, respectively, and the output should be the output of a list comprehension. I believe that a: List[T] and b: List[T] are probably too specific, and the return type List[T] is probably wrong as well.

I'm very new to this particular type system. I haven't yet figured out how to find out information like this. It's probably described somewhere in the documentation, so I'd also appreciate a brief description of how to find such documentation in the future.

Upvotes: 0

Views: 2098

Answers (1)

Barmar
Barmar

Reputation: 782285

The parameter you're iterating over should be Iterable, and the parameter you're searching should be Container.

from collections.abc import Iterable, Container

def dual_combinator(self, a: Iterable[T], b: Container[T]) -> List[T]:
    return [x for x in a if x in b]

The list comprehension always returns a list, so List[T] is the correct return type.

Upvotes: 3

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