Output coroutine functions

[Victor-Savu]

It would be great if @decorator could be used to create decorators which produce coroutine functions. Is there already such functionality and I just didn't find it? If not, would this be feasible? If so, are there any plans for this? Would you accept PRs?

Example:

from inspect import iscoroutinefunction
from decorator import decorator

@decorator
async def ensure_coro(function, *args, **kwargs): 
    ret = function(*args, **kwargs)
    if iscoroutinefunction(function):
        ret = await ret
    return ret

@ensure_coro
def foo():
    return 42

assert iscoroutinefunction(foo)

[micheles]

The goal of the decorator module is to provide a framework on top of which you can build useful decorators. It is not meant to be a library of decorators, so I cannot accept this kind of contributions. Still, if somebody wants to build a library of decorators on top of my module I am pretty happy with that ;-)

[Victor-Savu]

Thanks for the insights! I had a look at the code, mainly at the workhorse class that creates functions and I am not sure it can be used as is for generating coroutine functions, especially since the regular expression for verifying the function expects it to begin with def and not async def. So I am wondering if it would even be possible to leverage your library for creating such a decorator.

[micheles]

You raise an interesting point. I think that the decorator module should be made able to decorate async functions. I have not done that until now since I still need to support Python 2. But it is something I should think about and work on, when I will have the time. I am reopening the issue.

[Victor-Savu]

Thanks! Just to clarify: async functions are decorated more-or-less correctly since calling them produces an awaitable object. The problem is that in the process an important part of their signature is gone: the fact that they are coroutine functions.
I know that Python 2 compatibility requires a lot of jumping through hoops in order to provide support for new Python 3 features, so kudos on your awesome results!

[micheles]

I have just pushed in master a change that make decorators preserving the iscoroutinefunction property. Can you test it? It seems to be working with the async def syntax, but not with @asyncio.coroutine.
I do not have code using coroutines, but if you report that it works well I could make a new release 4.1.0
with this new feature.

[Victor-Savu]

Thanks for taking this up! The decorated function is now a coroutine function, but it returns another coroutine function, so two awaits are necessary:

from decorator import decorator
from asyncio import get_event_loop
from inspect import iscoroutinefunction

def awt(coro):
    """ A synchronous version for await """
    return get_event_loop().run_until_complete(coro)
 
@decorator
async def foo(coro, *args, **kwargs):
    return "<before>" + (await coro(*args, **kwargs)) + "<after>")

@foo
async def baba(ba):
    return ba

async def main():
    print(iscoroutinefunction(baba))  # prints: True

    print(await baba("banana"))  # prints: <coroutine object foo at 0x7f1dc04fc308>
                                 # and issues: RuntimeWarning: coroutine 'foo' was never awaited

    print(await (await baba("banana"))) # prints: <before>banana<after>

awt(main())

Before the change, one await was enough.

Regarding the @asyncio.coroutine decorator, it is perfectly acceptable since the decorated function would not satisfy iscoroutinefunction anyway:

In [9]: @asyncio.coroutine
   ...: def foo():
   ...:     a = yield from bar()
   ...:     return a
   ...: 

In [10]: iscoroutinefunction(foo)
Out[10]: False

[micheles]

I have fixed this in the current master. Let me know if you find other unexpected behaviors, I guess there are corner cases.

[Victor-Savu]

Thanks! I'll only be able to do a thorough review in the beginning of next week. Would that be OK?

[Victor-Savu]

I am sorry it took so long! I've had three rather busy weeks. However, I wanted to say that I think the implementation works great and I couldn't find any other problems with it. I am very happy with the solution and I want to thank you very much for your continued work!

[micheles]

Just FYI, today I have released a new official version of the decorator module supporting decorating coroutines. And it was all thanks to you! Please keep supporting the decorator module, I am back from the EuroPython and there are still people not using it when they should! ;-) Michele

…

On Mon, Jun 26, 2017 at 11:29 PM, Victor-Nicolae Savu < ***@***.***> wrote: I am sorry it took so long! I've had three rather busy weeks. However, I wanted to say that I think the implementation works great and I couldn't find any other problems with it. I am very happy with the solution and I want to thank you very much for your continued work! — You are receiving this because you modified the open/close state. Reply to this email directly, view it on GitHub <#41 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACWYcI_kSE63BSj6WiqZqFAB4HgEMtmQks5sICLNgaJpZM4Nq2UV> .

[Victor-Savu]

Thanks! I hope you had a nice time at EuroPython! I'll keep an eye open for the recorded sessions on YouTube.