Possible way to type hint callbacks

[matperc]

Right now we hint all the callables that admits variable arguments as Callable[..., T]. Starting from python 3.10 we can use the combination of Concatenate and ParamSpec. Link to documentation.

So for example GAsyncReadyCallback can be typed as

__P__ = ParamSpec("__P__")

Callable[Concatenate[GObject.Object, Gio.AsyncResult, __P__], None]

@lovetox there are any plans to support 3.10?

[lovetox]

Its not about supporting 3.10 rather about not supporting everything that was before, except we cover everything with conditionals.

Python 3.10 is now in debian stable i think we could drop everything else.

Type hints are usually only used for CI and development, so i see no reason why people would need to run type checkers with old python versions.

especially because type checkers have the option to specifically test against older python versions anyway

[matperc]

Good, I will work on this

[demberto]

While ParamSpec and Concatenate both are added to typing in 3.10, they exist in typing_extensions which is available for all Python versions since 3.7.

[matperc]

While ParamSpec and Concatenate both are added to typing in 3.10, they exist in typing_extensions which is available for all Python versions since 3.7.

Thanks for the info. I will use this then

[matperc]

While with pyright there aren't errors, mypy throw this:

Argument 3 to "read_async" of "File" has incompatible type "Callable[[Optional[Object], AsyncResult], None]"; expected "Optional[Callable[[Optional[Object], AsyncResult, VarArg(<nothing>), KwArg(<nothing>)], None]]"  [arg-type]

The error doesn't make any sense to me, I suppose that it's not correct to use this method for this use case.

[demberto]

Looks correct to me for the most part, the callback argument takes in a Callable or None, hence its optional. I don't understand the VarArg, KwArg part, maybe its the user_data argument.

Also, if you don't care about type hinting for Python < 3.7, you could simply add

from __future__ import annotations

and replace:

Optional[str]

with the newer syntax without breaking compatibility:

str | None

[matperc]

It seems that the problem lays on Optional

from typing import Concatenate, ParamSpec, Callable


P = ParamSpec("P")

def func1(func: Callable[Concatenate[str, P], None], *args):
    pass

def func2(a: str, b: str):
    pass

func1(func2, "hello")

This works

from typing import Concatenate, ParamSpec, Callable, Optional


P = ParamSpec("P")

def func1(func: Optional[Callable[Concatenate[str, P], None]], *args): # Optional func here
    pass

def func2(a: str, b: str):
    pass

func1(func2, "hello")

This not

A bug in mypy?

[demberto]

A bug in mypy?

Yes probably.

[lovetox]

did you try other type checkers like for example pyright?

[matperc]

did you try other type checkers like for example pyright?

With pyright works fine. In the PR I linked an issue that is similar. I have added some comments, let's see if someone is able to fix it

[lovetox]

The question for me is, you wrote the MR is blocked by that mypy issue.

Personally for me thats not blocking, there exist at least one type checker who does it right.

mypy has 2.4 k open issues, what if they never fix that issue?

[matperc]

The question for me is, you wrote the MR is blocked by that mypy issue.

Personally for me thats not blocking, there exist at least one type checker who does it right.

mypy has 2.4 k open issues, what if they never fix that issue?

I see your point. Maybe let's wait some time to see if someone came up with a solution

[lovetox]

Yeah no hurry, your work, your MR, your decision :)
Just wanted to point out that there is more than mypy.