Function "builtins.callable" is not valid as a type

[ramalho]

Bug Report

Mypy does not recognize the built-in function callable as a valid type for type hinting.

In the spirit of PEP 585 -- Type Hinting Generics In Standard Collections, it should.

I understand callable is not subscriptable, and I would file a bug in BPO if you agree this issue is worth fixing.

Nevertheless, I can't use callable even to typecheck very basic code like this the example below.

To Reproduce

1. Create a module with this:

def f(g: callable) -> None:
    g()

f(f)
f('a')

2. Type check it:

$ mypy callable_type.py 
callable_type.py:3: error: Function "builtins.callable" is not valid as a type
callable_type.py:3: note: Perhaps you need "Callable[...]" or a callback protocol?
callable_type.py:4: error: callable? not callable
Found 2 errors in 1 file (checked 1 source file)

Expected Behavior

From a user perspective, the distinction between callable and abc.Callable (or typing.Callable) is as arbitrary as the distinction between list and typing.List, which PEP 585 finally fixed.

I believe Mypy should accept callable in type hints, and that Python should make callable implement __class_getitem__. If it's not a class, it should become one (this has happened before in the history of Python; it's a non-breaking change if done correctly).

Actual Behavior

Mypy refuses to accept builtins.callable in a type hint.

Your Environment

• Mypy version used: 0.818
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.4
• Operating system and version: MacOS 10.14.6.

[JelleZijlstra]

There is an existing issue about making callable generic at runtime: https://bugs.python.org/issue42102.

If we implemented your proposal, we'd also have to teach --disallow-any-generics to tell people to give type parameters to callable, and then they'd be back to using Callable. So we'd be introducing a new feature and simultaneously tell people to not use it. As such, I don't think it makes sense for mypy to accept callable as a type until callable[...] is accepted at runtime.

[ramalho]

Your answer makes perfect sense, thanks @JelleZijlstra.

I did not know about https://bugs.python.org/issue42102 and I agree we should wait for it to be solved.

[hauntsaninja]

Yeah, this should be tracked at python/cpython#86268. I think currently, people don't really want to make this change.