Add more precise error message for Callable annotation

[frerikandriessen]

Description

Adds a more precise error message for a Callable annotation, if the
option --disallow-any-generics (part of --strict mode) has been used. The suggestion in the old
message was incorrect in this scenario.

Fixes #11757

Test Plan

Contents of test.py:

from typing import Callable

def my_function(callback: Callable[...]) -> None:
    callback()

Results:

$ mypy test.py --disallow-any-generics
test.py:3: error: Please use "Callable[[<parameters>], <return type>]" or "Callable[..., Any]"
Found 1 error in 1 file (checked 1 source file)
$ mypy test.py
test.py:3: error: Please use "Callable[[<parameters>], <return type>]" or "Callable"
Found 1 error in 1 file (checked 1 source file)

[JelleZijlstra]

What if we just remove the "or Callable" part? We should nudge people towards more precise types, and error messages don't need to include every detail.

[AlexWaygood]

What if we just remove the "or Callable" part? We should nudge people towards more precise types, and error messages don't need to include every detail.

We could probably remove the "or Callable" part if a user has the --disallow-any-generics flag set to True. I don't think that would be a good idea if a user doesn't have that flag set, though. I think we should support gradual typing as much as possible, and Callable is one of the least ergonomic types when it comes to parameterization.

[frerikandriessen]

One of my least favorite parts of typing is typing a Callable that has a very intricate call signature. Most of the times, the Callables that are very complex to type are Callables that I do not even directly use in my code but have to type to satisfy mypy. It's nice to, in those cases, be able to just use Callable[..., Any] to explicitly indicate the specific typing is not important.

That said, I can see an argument being made that, if you choose for the --strict flag, you want to only allow the strictest way. But then why allow the shorter form in this mode anyway?
With regards to nudging, I'd say that if you have --strict mode enabled (I'm assuming this is the case for most situations where --disallow-any-generics is enabled), you are already intent on doing the "right" thing; hiding this information would just make it more annoying to do so.

Dependent on the outcome of this discussion I will fix the flake8 error or change the error message.

[JelleZijlstra]

@frerikandriessen what flake8 error are you referring to?

And note that I'm not proposing to remove the ability to write "Callable[..., Any]", just to make the error message more concise in this case.

[AlexWaygood]

@frerikandriessen what flake8 error are you referring to?

The flake8 CI check is failing because one of the lines is slightly too long

[JelleZijlstra]

Suggested change

	self.fail('Please use "Callable[[<parameters>], <return type>]" or "Callable[..., Any]"', t)
	self.fail('Please use "Callable[[<parameters>], <return type>]"', t)

Let's remove the suboptimal suggestion here

[frerikandriessen]

My 2 cents: I actually want to keep the error message with both suggestions, because the 'suboptimal' suggestion is exactly what I wanted to know. So for me it was very helpful (albeit unfortunately misleading, which is why I've corrected it). Just to emphasize that some users will get value from the not-concise message.

That said, if you still want me to remove that part I'll gladly do so.

[frerikandriessen]

@JelleZijlstra just to double-check: I lined out above why I wanted to keep that message as is. Do you still want me to change it?

[JelleZijlstra]

I'd still lean towards removing it. Error messages should be short and provide the most useful option, and Callable[..., Any] generally isn't what we should steer people towards.

[frerikandriessen]

Okay, I've incorporated your suggestion

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉