Support narrowing Union of List types via index and isinstance

[sbdchd]

It would be nice to be able to narrow a union of two different list types via indexing into the array and using an isinstance check

from typing import Union, List

x: Union[List[str], List[int]] = [10]

if isinstance(x[0], str):
    reveal_type(x)
    # expected: builtins.list[builtins.str]
    # actual: Union[builtins.list[builtins.str], builtins.list[builtins.int]]
else:
    reveal_type(x)
    # expected: builtins.list[builtins.int]
    # actual: Union[builtins.list[builtins.str], builtins.list[builtins.int]]

One caveat is that the array could be empty, but let's assume we make an emptiness check before indexing

[gvanrossum]

This is a special case in mypy. If you rewrite the initialization like this it works:

x: Union[List[str], List[int]]
x = [10]

Honestly I don't think this is documented very well -- you might be able to infer this from Explicit types for variables but it could be more explicit.

[sbdchd]

I think I may have made my example too simple.

In my current use case I parse some data from a network response which is a union of two different list types: Union[List[List[str]], List[SearchResult]]

I'm hoping to refine this response via indexing and an isinstance check

from typing import Union, List
from dataclasses import dataclass


@dataclass
class SearchResult:
    id: str
    score: float
    match: str


def get_search_results() -> Union[List[List[str]], List[SearchResult]]:
    ...


x = get_search_results()

if isinstance(x[0], SearchResult):
    reveal_type(x)
    # expected: builtins.list[main.SearchResult]
    # actual: Union[builtins.list[builtins.list[builtins.str]], builtins.list[main.SearchResult]]
else:
    reveal_type(x)
    # expected: builtins.list[builtins.str]
    # actual: Union[builtins.list[builtins.list[builtins.str]], builtins.list[main.SearchResult]]

which I think can be simplified to the following as a test case

from typing import Union, List


def foo() -> Union[List[str], List[int]]:
    ...


x = foo()

if isinstance(x[0], str):
    reveal_type(x)
    # expected: builtins.list[builtins.str]
    # actual: Union[builtins.list[builtins.str], builtins.list[builtins.int]]
else:
    reveal_type(x)
    # expected: builtins.list[builtins.int]
    # actual: Union[builtins.list[builtins.str], builtins.list[builtins.int]]

[gvanrossum]

Ah, okay. I think this is a feature request.

[hauntsaninja]

Duplicate of #7339 and #2458