TypeVar bound to Union in if isinstance(...) branch has incompatible return type

Bug Report

A TypeVar bound to Union[x, y] will resolve to x after an if isinstance(..., x) is used which is incompatible with the return type.

I'm not sure on how to describe exactly what is going on, better check the Example.
Also mypy experts, please forgive me if that is intended behavior and not a bug (I would appreciate a working solution in this case :)

To Reproduce

Example:

from typing import TypeVar, Union

T = TypeVar("T", bound=Union[str, int])

def a(x: T) -> T:
    if isinstance(x, str):
        return x
    return x

Expected Behavior

I expect that this definition is correct, since whatever input is given, the output is the same.

Actual Behavior

mypy raises error: Incompatible return value type (got "str", expected "T").

Further Info

I want to have the str -> str, int -> int mapping, so simply using Union[str, int] won't work.
This function is called sometimes with variables of type Union[str, int], so using TypeVar("T", str, int) won't work either.

Your Environment

Mypy version used: 0.950
Mypy command-line flags: --strict
Mypy configuration options from mypy.ini (and other config files):
Python version used: 3.9
Operating system and version: win10

As a side note: a reveal_type(x) in the else branch returns "T`-1". Maybe a more expressive type would be useful?

This one looks like a bug to me :)

For a (horrible) workaround, you can use overloads instead:

from typing import overload

@overload
def a(x: str) -> str: ...

@overload
def a(x: int) -> int: ...

@overload
def a(x: int | str) -> int | str: ...

def a(x: int | str) -> int | str:
    if isinstance(x, str):
        return x
    return x

b: str
c: int
d: int | str

reveal_type(a(b))  # Revealed type is "builtins.str"
reveal_type(a(c))  # Revealed type is "builtins.int"
reveal_type(a(d))  # Revealed type is "Union[builtins.int, builtins.str]"

I don't know what does it mean for debugging, but this is also a bit odd:

from typing import TypeVar, Union

T = TypeVar("T", bound=Union[str, int])

def a(x: T) -> T:
    reveal_type(x)  # T`-1
    if isinstance(x, str):
        reveal_type(x)  # str
        return x
    reveal_type(x)  # T`-1
    return x

Gives T`-1, str and T`-1 revealed (in order) and error on line 9 (1st return). However, the following:

from typing import TypeVar, Union

T = TypeVar("T", bound=Union[str, int])

def a(x: T) -> T:
    reveal_type(x)  # T`-1
    if isinstance(x, str | None):
        reveal_type(x)  # T`-1
        return x
    reveal_type(x)  # T`-1
    return x

Gives T`-1 three times and no error. But I just add None which is not a member of T bounds, so it shouldn't affect inference, right?.. Say, the following ignores None in similar case:

x: str
if isinstance(x, str | None):
    reveal_type(x)  # str

But... Maybe it should be a separate issue, given this weird result with mypy from master and python 3.10:

x: str | int
if isinstance(x, str | None):
    reveal_type(x)  # N: Revealed type is "Union[builtins.str, builtins.int]"

@headtr1ck

@headtr1ck, a slightly simpler and less horrible workaround: just use a cast():

from typing import TypeVar, Union, cast

T = TypeVar("T", bound=Union[str, int])

def a(x: T) -> T:
    if isinstance(x, str):
        return cast(T, x)
    return x

@headtr1ck

@headtr1ck, a slightly simpler and less horrible workaround: just use a cast():

This is what I ended up doing in the moment.

Is this a duplicate of #10817?

Closing as duplicate of #10817

Closing as duplicate of #10817

#10817 is closed but the problem persists in python 1.10.1

#10817 is closed

not true, it is still open

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`TypeVar` bound to `Union` in `if isinstance(...)` branch has incompatible return type #12800

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TypeVar bound to Union in if isinstance(...) branch has incompatible return type #12800

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headtr1ck commented on May 16, 2022

AlexWaygood commented on May 16, 2022

sterliakov commented on May 17, 2022

AlexWaygood commented on May 17, 2022

headtr1ck commented on May 17, 2022

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AlexWaygood commented on Jun 25, 2024

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`TypeVar` bound to `Union` in `if isinstance(...)` branch has incompatible return type #12800