singledispatch and TypeVar return type

[vnmabus]

It seems that singledispatch typing is not very accurate. In particular, it does not respect TypeVar used in the return type:

T = TypeVar("T")

@singledispatch
def my_fun(
    val: T,
) -> List[T]:
    return list(val)

my_fun(5)
my_fun("Ĝis la revido!")

Here the type of the returned value would always be revealed as List[T] instead of different types for different inputs.

[srittau]

Unfortunately, I don't see a way to express this using the current type system. This is possibly something where PEP 646 can help with. For now, I will marks this as "inexpressable" and close it, but we can revisit this if and when a solution becomes available in the type system.

[srittau]

Or possibly PEP 612, for which we have a tracker issue: #4827.

[srittau]

Reopening as PEP 612 is now mostly available.