Why does mypy not accept a list[str] as a list[Optional[str]]?

Question

Example 1:

from typing import List, Optional

def myfunc() -> List[Optional[str]]:
    some_list = [x for x in "abc"]
    return some_list

Mypy complains on example 1:

Incompatible return value type (got "List[str]", expected "List[Optional[str]]")

However, this example gets no complaint:

Example 2:

def myfunc() -> List[Optional[str]]:
    some_list = [x for x in "abc"]
    return list(some_list)

What is the explanation for the inconsistent behavior?

Alex Waygood · Accepted Answer · 2021-10-07 13:13:22Z

6

Since in Python lists are invariant (see the examples here and here).

If we pass List[str] to someone that expects List[Optional[str]], that someone may add a None to our list and break our assumptions. The second example is valid however as the output of list() in the return statement is not saved anywhere and no one can depend on the the returned value being mutated illegally .

edited Oct 7, 2021 at 13:13

Alex Waygood

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answered Oct 7, 2021 at 10:30

lorg

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3 Comments

joel Over a year ago

list invariance is sure part of the story, but i'd also expect python to be able to infer the types here regardless. I consider it a bug that mypy's too eager in inferring the type of some_list as list[str]

kaya3 Over a year ago

@joel I'd consider that a feature request, and it's not obvious what the behaviour should be. There would be similar examples to this code where you want List[str] for the intermediate variable.

joel Over a year ago

@kaya3 yes you're right it is a feature request. I went as far as calling it a bug because I feel it's so important to user experience

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