type refinement in function call

[dfroger]

This is ok for mypy:

from typing import Union

def foo(x: Union[int, str]):
    if isinstance(x, int):
        return x + 10

but how can I make it work if the isinstance test is in a function call?

from typing import Union

def check(x: Union[int, str]) -> bool:
    return isinstance(x, int)

def foo(x: Union[int, str]):
    if check(x):
        return x + 10

I get:

foo.py:8: error: Unsupported operand types for + ("Union[int, str]" and "int")

[JelleZijlstra]

This could perhaps be done with literal types in the future, something like this:

@overload
def check(x: int) -> Literal[True]: ...
@overload
def check(x: str) -> Literal[False]: ...

[gvanrossum]

That's pretty verbose though..

…

On Tue, Jun 12, 2018, 08:08 Jelle Zijlstra ***@***.***> wrote: This could perhaps be done with literal types in the future, something like this: @overload def check(x: int) -> Literal[True]: ... @overload def check(x: str) -> Literal[False]: ... — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#5206 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACwrMvKHVxoXmkkO8mpf8aeuMzX0ZZmTks5t79lfgaJpZM4UkjNs> .

[gwerbin]

I've had this issue recently too. I definitely think Mypy would benefit from some way to refine types by following conditional paths. Unfortunately implementing that kind of logic is well beyond my expertise.

AFAIK you would still have the problem if you wrote if True instead of if check(x), so having literal return types probably won't fix this specific problem.

[elazarg]

Related: #1203, #2357 #4063, #3062

[ilevkivskyi]

Here I copy the relevant link from one of the above issues I just closed microsoft/TypeScript#1007