Allow usage of functools.partial async handlers

[vladmunteanu]

At the moment, one cannot pass a partial asynchronous handler, because:

1. they will be recognized as ASGI and treated as such.
2. asyncio.iscoroutinefunction fails to detect a partial in Python versions < 3.8 Stackoverflow discussion

[erewok]

This is interesting. I would like to be able to use partial functions as handlers as well and didn't know about this bug. Two questions:

• In the stackoverflow question you posted, the answer there descends into the partial function using a while loop (included below)¹. It states that this is the solution added to Python 3.8. Do we also need to be able to handle more than one layer of partial-wrapping?

• Since this bug affects Python versions less than 3.8, should we check the version and only run this check for earlier versions (with the idea that Starlette could eventually stop supporting earlier versions after enough time has passed)?

1. Example function from the Stackoverflow answer:

def iscoroutinefunction_or_partial(object):
    while isinstance(object, functools.partial):
        object = object.func
    return inspect.iscoroutinefunction(object)

[vladmunteanu]

@erewok I think my eyes skipped right over that while statement when I initially read the issue.
I think we should definitely check for multiple levels of wrapping too, and I agree we can add a check for py versions and only process this on py < 3.8.

If this sounds good, I'd like to add the method you pointed out (iscoroutinefunction_or_partial), with an extra check for the py versions.

[erewok]

I like those ideas. I would defer to @tomchristie on merging the result.

[vladmunteanu]

@erewok I was afraid of this... with the py 3.8 check, we now fall under 100% coverage on that interpreter. Any suggestions? Or should we just dump the check and let the while loop run?

[erewok]

Let's see what @tomchristie thinks about this PR. I'm not sure how he'll want to handle different behavior in different Python versions and don't want to suggest you do a bunch of work. I think the idea is solid overall.

[vladmunteanu]

BTW, this is the kind of usage i'm thinking about:
https://github.com/vladmunteanu/starlette-jsonapi/blob/a35dbb6ae5f15d4ce9ccdaad01751445d743f32b/starlette_jsonapi/resource.py#L173

I had to write some methods (ie: handle_get, handle_post) that otherwise might not be needed if partials would be possible.

[kevinastone]

@vladmunteanu You can add a # pragma: no cover to the python version check line to ignore that workaround for code coverage purposes.

Example in my recent PR:
https://github.com/encode/starlette/pull/1007/files#diff-f43e94ab436a8cbb273305cbe72b36e3R38

[vladmunteanu]

@kevinastone thanks, I was going to propose that, but I was also curious what @tomchristie has to say about this before. I guess I should just do it to increase my chances.

[vladmunteanu]

Hey @erewok , thanks for the approval!
Any idea when it will be released?

[erewok]

hello @vladmunteanu, sorry for the late reply, but I haven't been available for open source for months due to work obligations. I will check with the encode maintainers this week to see if everyone is okay with releasing this. Thanks again.

[florimondmanca]

Just checked out this PR and this looks good to me. :)

[vladmunteanu]

Thanks everyone for getting this through!