bpo-46414: Add typing.reveal_type

[JelleZijlstra]

Part of the documentation was taken from mypy:
https://mypy.readthedocs.io/en/stable/common_issues.html#reveal-type

https://bugs.python.org/issue46414

[sobolevn]

Thank you! This solves my old pain with redefining reveal_type as:

from typing import TYPE_CHECKING
if not TYPE_CHECKING:
    reveal_type = print

[gvanrossum]

Hold on. This feels awkward, since previously this was a builtin. I don’t want to have to import it.

[JelleZijlstra]

Hold on. This feels awkward, since previously this was a builtin. I don’t want to have to import it.

Type checkers would still allow using it without the import. You'll have the option to import it if you want to run your code or document to readers what reveal_type() means.

[gvanrossum]

It needs more discussion. Please wait until I have a keyboard.

[JelleZijlstra]

No hurry! I also opened a thread on typing-sig about this, that's probably the best place to discuss how it should work.

[JelleZijlstra]

@gvanrossum based on the discussion on typing-sig, are you satisfied with this change now?

I'm planning to also add reveal_locals() in a separate PR.

[JelleZijlstra]

Wait, I didn't address Nikita's review yet.

[gvanrossum]

Did we bikeshed yet whether it should write to stdout or stderr?

[JelleZijlstra]

Did we bikeshed yet whether it should write to stdout or stderr?

No, though there have been some suggestions about the output format; I just sent a typing-sig email about that too. Personally I don't care much but I'd like to keep it simple.

[gvanrossum]

Did we bikeshed yet whether it should write to stdout or stderr?

No, though there have been some suggestions about the output format; I just sent a typing-sig email about that too. Personally I don't care much but I'd like to keep it simple.

I'm just thinking that sending the output to stderr would be more in line with the debug nature of the API.

[JelleZijlstra]

Pushed a change to make it write to stderr instead.

[sobolevn]

Do we use type hints in typing.py? 🤔

[JelleZijlstra]

If this is merged, we will :). I don't see any reason not to, especially in cases like this where the type is simple and helps document the behavior.

[AlexWaygood]

There are type annotations in a few lines of typing.py:

cpython/Lib/typing.py

Line 2531 in 270a091

def read(self, n: int = -1) -> AnyStr:

It's just about the only module in the stdlib in which I've spotted any type annotations other than importlib :)

[JelleZijlstra]

@gvanrossum do you think this is ready to merge now? The discussion on typing-sig has died down. (You added the "Do not merge" label a while ago.)

I think the other new function PRs are also ready, but I can ping you on those later.

[gvanrossum]

LGTM.

[bedevere-bot]

@gvanrossum: Please replace # with GH- in the commit message next time. Thanks!

[gvanrossum]

Should probably post the resolution to typing-sig.