Add ReturnType-like utility for getting function return types

[strokirk]

This is a feature request, so if there is a way to achieve this already, please point me and future me in the right way.

In TypeScript, we can use the ReturnType utility type to get the return type of a function to reuse in other places. This is very useful to make the code short, readable, and easy to change if necessary, especially in cases where I want to wrap the call of a function from another module in some other function.

An example of how it might be used:

# module: foo.py
def foo() -> None | baz.Baz | tuple[int, int]:
    ...

# module: bar.py
def bar() -> ReturnType[foo.foo]:
    ...

In this case, I could maybe give the return type a name like FooReturn and import that in bar, but that seems less elegant rather than just reusing the function I'm already using.

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Note that if automatic or manual type inference (#4409) was implemented, most of my usages would be covered.

[JelleZijlstra]

PEP 612 (https://www.python.org/dev/peps/pep-0612/) may cover some of your use case.

[intgr]

If both foo.py and bar.py live in the same project, you can create a type alias and use that multiple times. I find that it makes code clearer too.

[JukkaL]

I can see how this would be useful, at least occasionally. However, this is not easy to implement in full generality, primarily because of decorators that can change the signature of a function. One option would be to only support functions with simple decorators as targets.

We'd be more likely to prioritize this if there was at least a draft PEP about the feature.

[strokirk]

I think if a PEP were to be written, it would be nice to consider more general type introspection, from which a ReturnType-like utility could be built, as well as other useful utilities.

Thanks for the response!