enumerate causes incompatible type mypy error

[jroberts07]

Bug Report

When using the enumerate function mypy seems to generalise the output to object instead of respecting the type of the input

To Reproduce

from typing import Union


def process(actions: Union[list[str], list[int]]) -> None:
    for pos, action in enumerate(actions):
        act(action)


def act(action: Union[str, int]) -> None:
    print(action)

Expected Behavior

Mypy raises no errors

Actual Behavior

Mypy raises the following error: Argument 1 to "act" has incompatible type "object"; expected "Union[str, int]"

Its worth noting that the same code without using enumerate does not raise this error:

from typing import Union


def process(actions: Union[list[str], list[int]]) -> None:
    for action in actions:
        act(action)


def act(action: Union[str, int]) -> None:
    print(action)

Your Environment

• Mypy version used: 0.921
• Python version used: 0.9

[sobolevn]

You can try this (if you are using python>=3.9):

from typing import Union

def process(actions: Union[list[str], list[int]]) -> None:
    for pos, action in enumerate[str | int](actions):
        act(action)

def act(action: Union[str, int]) -> None:
    print(action)

But, I agree that automatically infering str | int here can be a good feature. But, it is really hard to do 🙂

[erictraut]

This is yet another case where mypy's choice to use join rather than union causes problems. Pyright uses only unions, never joins, so it produces the expected result in this case.

Has there been any serious discussion of changing mypy's behavior to use unions instead of joins? I can't think of any case where joins are preferable, and there are many cases where unions are better because they retain more detailed type information.

[sobolevn]

CC @JukkaL

[AlexWaygood]

Closing as a duplicate of #8586