PEP 718: subscriptable functions

[Gobot1234 (James Hilton-Balfe)]

Thanks for the comments and sorry I took so long to get to this.

@Jelle

I guess what you’re saying is that the PEP becomes useful if there was a way to access the type argument at runtime. But the current version of the PEP doesn’t provide for that.

Yeah I don’t entirely disagree with this sentiment though I do think it does have merit to stand on its own feet as outlined at the end.

@Jelle

Empty containers

def make_list[T](*args: T) -> list[T]: ...
reveal_type(make_list[int]())  # type is list[int]

This makes sense for the specific case of empty containers. Why would you write such a function, though? Are there examples of real users who needed this?

Some of the examples from the PEP were taken from real world code this wasn’t one of these pieces of code though I don’t think it’s actually that unreasonable to see a case where this could come up

@Jelle

On the other hand, there is a trivial workaround that works today: you can write my_foo: Foo[int] = factory(lambda x: "Hello World" * x). Guido rightly points out above that this may be less convenient if the type is more complicated than just Foo[int]. Still, how common is this sort of thing in practice? Can you point to e.g. mypy issues where someone writing real code would have been helped by this?

Yes this came up from a library online that a friend is writing GitHub - Zomatree/Kine: React like agnostic GUI framework for Python

I’m curious if you’d be able to dig these up because they’d be useful for adding strength to the argument.

My main reasons for writing the PEP

• Completely agree with Gudio’s point about the complexity of the types, I think this feeds into the next point nicely.
• Symmetry with classes. I’ve had multiple people be surprised that this wasn’t already a feature. I think it is a logical change post PEP 585.
• Prospect of runtime access. I know that this isn’t really brought up in the PEP though it is something I am working on, I did see this however as a large enough change that is should be separated from this PEP.