bpo-46480: add typing.assert_type

[JelleZijlstra]

Another pending typing-sig proposal. This function serves as a static assertion to the type checker about the type of a value.

https://bugs.python.org/issue46480

[JelleZijlstra]

@gvanrossum this one is also ready I believe. Feedback on this function in https://mail.python.org/archives/list/typing-sig@python.org/thread/MITFQ6Z45RRMXY3HNM66IC3XXS3TA3JN/#MITFQ6Z45RRMXY3HNM66IC3XXS3TA3JN was positive. (I also proposed assert_error() there, but dropped it because of negative feedback.)

[davidfstr]

It occurs to me that users of "assert_type()" may be surprised that it doesn't actually perform an assertion at runtime.

Consider code like:

from typing import assert_type

def parse_int(value) -> int:
    # Hmm. A casual observer would likely read this incorrectly as `assert isinstance(value, str)`...
    assert_type(value, str)
    return int(value)

I wonder if we might consider a word other than "assert", or maybe add the word "static" somewhere. Suggestions:

• prove_type(value, str)
• static_assert_type(value, str)

[JelleZijlstra]

That's a reasonable concern, but it's probably better discussed on the typing-sig thread. I'm happy to change the PR if there's consensus for another name. I'll note though that cast() has the same potential problem, and I haven't heard of confusion because of that.

[davidfstr]

it's probably better discussed on the typing-sig thread

I'll plan to make a post tomorrow.

[JelleZijlstra]

@gvanrossum I think this should be ready to merge soon, since people on the typing-sig thread now seem on board with the assert_type name. I'd appreciate some more reviews for the documentation though, maybe @Fidget-Spinner or @AlexWaygood.

[AlexWaygood]

@gvanrossum I think this should be ready to merge soon, since people on the typing-sig thread now seem on board with the assert_type name. I'd appreciate some more reviews for the documentation though, maybe @Fidget-Spinner or @AlexWaygood.

I'll try to take a look soon!

[AlexWaygood]

Thanks! Looks pretty good, just a few thoughts :)

[JelleZijlstra]

Thanks for the review!

[AlexWaygood]

LGTM!

[davidfstr]

Just 1 tiny nit:

[JelleZijlstra]

Planning to merge this tomorrow unless I get more feedback.