Unexpected error when narrowing TypeVar

[pganssle]

Bug report

I ran into this recently and @glyph came up with a very elegant minimal working example:

from typing import TypeVar

T = TypeVar("T")

def f(s: T) -> T:
    if isinstance(s, str):
        return s # main.py:7: error: Incompatible return value type (got "str", expected "T")
    return s

Basically, if you have a function that takes a typevar and you narrow it using an isinstance check, mypy decides that the concrete type does not match the generic type from the signature, despite the fact that the concrete type is just the narrowed generic.

The best workaround I've found is to return cast(T, s). Glyph and I both tried playing around with overloads and couldn't find something to work around the issue other than just a cast.

[glyph]

I should note that there are some other cases which make this complicated:

from typing import TypeVar, reveal_type

T = TypeVar("T")

def f(a: T, b: T) -> T:
    return a

reveal_type(f("1", 2))

T is object in this call, due to that being the common supertype of the two arguments. So the TypeVar itself, T, is actually widened, to include str but not necessarily to exclude anything else. For example, if isinstance(a, str) is true, we can say nothing about b; but we can say that another str should be an acceptable return type.

Ooh. Except, maybe not:

from typing import TypeVar, reveal_type, NewType

T = TypeVar("T")

Special = NewType("Special", str)
TheSpecial = Special("Hello")

def f(a: T) -> T:
    if isinstance(a, str):
        return "Goodbye"
    return a

reveal_type(f(TheSpecial))

If "Hello" is the only Special string allowed, then the runtime check tells us nothing here.

Maybe this is not trivially possible.

[sterliakov]

Is it the same issue as #13956?

[glyph]

Is it the same issue as #13956?

I believe so.

[hauntsaninja]

Closing as duplicate of #10817