bpo-40816 Add AsyncContextDecorator class

[heckad]

https://bugs.python.org/issue40816

[eamanu]

IMHO, we need test here, docs.. I thing this need more discussion?

[heckad]

Do you know you can write something like

@contextmanager
def safe_space(is_on=True):
token = IS_SAFE_SPACE_VALUE.set(is_on)
try:
yield
finally:
IS_SAFE_SPACE_VALUE.reset(token)

And use it as

class MyClass:
@safe_space()
def safe_method(self):
pass

but you can't write

@asynccontextmanager
async def async_safe_space(is_on=True):
token = IS_SAFE_SPACE_VALUE.set(is_on)
try:
yield
finally:
IS_SAFE_SPACE_VALUE.reset(token)

because you get the error TypeError: '_AsyncGeneratorContextManager' object is not callable. This pull request makes '_AsyncGeneratorContextManager' callable. This is an implementation of lost functionality.

[heckad]

@1st1?

[njsmith]

@belm0 asked me to look at this, but tbh I'm not real comfortable with adding new API surface to contextlib without input from the module owners, which I think is @ncoghlan and @1st1.

The idea seems straightforward enough. @eamanu is right though that it definitely needs docs and tests.

[heckad]

@njsmith, I will add tests if this pull request is interesting. I was just waiting for a response from @1st1.

[belm0]

@heckad since core review attention is a scarce resource, I suggest having the PR ready to approve. Since the tests and docs can mostly be copied from ContextDecorator, not too much work?

This functionality is needed-- let's assume the maintainers will agree.

[heckad]

@belm0 How to improve the documentation?

[belm0]

@belm0 How to improve the documentation?

See context manager docs:

contextmanager() uses ContextDecorator so the context managers it creates can be used as decorators as well as in with statements. When used as a decorator, a new generator instance is implicitly created on each function call (this allows the otherwise “one-shot” context managers created by contextmanager() to meet the requirement that context managers support multiple invocations in order to be used as decorators).

(it would also need "Changed in version X.X" annotation)

Also see the ContextDecorator docs. Perhaps it's enough to say "Similar to ContextDecorator(), but works with async context managers" and give a short example.

https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager
https://docs.python.org/3/library/contextlib.html#contextlib.ContextDecorator

[1st1]

Oh wait! The code/tests look great! But we also need to update the docs before we merge this.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[heckad]

Need to add a version. Which version of python will be added this feature?

[1st1]

Need to add a version. Which version of python will be added this feature?

3.10.

[heckad]

Need to add a version. Which version of python will be added this feature?

3.10.

What about 3.9?

[1st1]

We missed that train, unfortunately. 3.9 is frozen for new features.

[heckad]

@1st1, all changes were applied.

[asvetlov]

Thanks @heckad