False-positive deprecation warning for partial async handlers

[nicktimko]

Long story short

Wrapping a coroutine function with a functools.partial causes a DeprecationWarning to trigger.

Steps to reproduce

Something like

def private_route(handler):
    return functools.partial(handler, private=True)

async def hello(request, private: bool):
    return web.Response(text='boo' if private else 'hello')

handler = private_route(hello)
app.router.add_route('get', '/hello', handler)  # DepWarning

causes a DeprecationWarning to be hit.

I think when called as handler(req) will immediately run through the partial and return the coroutine, which can then be awaited. It seems like the dep warning in this case is more of a false-positive. Maybe an expression like the ending one below could suppress it? If this is indeed just a false-positive and I'm the only complainer reporter, I could live.

import functools
import inspect

async def f():
    pass

fp = functools.partial(f)

inspect.iscoroutinefunction(f)  # True
inspect.iscoroutinefunction(fp) # False
isinstance(fp, functools.partial) and inspect.iscoroutinefunction(fp.func) # True

Tweak?:

         if asyncio.iscoroutinefunction(handler):
             pass
         elif inspect.isgeneratorfunction(handler):
             warnings.warn("Bare generators are deprecated, "
                           "use @coroutine wrapper", DeprecationWarning)
+        elif (isinstance(handler, functools.partial) and
+              asyncio.iscoroutinefunction(handler.func)):
+            pass
         elif (isinstance(handler, type) and
               issubclass(handler, AbstractView)):
             pass
         else:
             warnings.warn("Bare functions are deprecated, "
                           "use async ones", DeprecationWarning)

Your environment

I'm using aiohttp 3.3.2 on Python 3.6.5 on Ubuntu 16.04, but it looks like master would still trigger the warning.

[webknjaz]

Feel free to send a PR

[asvetlov]

The issue is essentially the same as #1993 (comment)
I doubt if introspection errors should be fixed on aiohttp level.

[nicktimko]

I guess I'm curious if my read was correct though: when synchronous handler support is removed, would this synchronous coroutine-handler-generator still work fine?

[asvetlov]

Are you asking for functools.partial?
No, partials registration will raise an exception

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new