Python asyncio: function or coroutine, which to use?

Question

I'm wondering if there are any noticeable differences in performance between foo and bar:

class Interface:
    def __init__(self, loop):
        self.loop = loop

    def foo(self, a, b):
        return self.loop.run_until_complete(self.bar(a, b))

    async def bar(self, a, b):
        v1 = await do_async_thing1(a)
        for i in range(whatever):
            do_some_syncronous_work(i)
        v2 = await do_async_thing2(b, v1)
        return v2

async def call_foo_bar(loop):
    a = 'squid'
    b = 'ink'
    interface = Interface(loop)
    v_foo = interface.foo(a, b)
    v_bar = await interface.bar(a, b)

But will the use of run_until_complete cause any practical, noticeable different to running my program?

(The reason I ask is I'm building an interface class which will accommodate decomposable "back-ends" some of which could be asynchronous. I'm hoping to use standard functions for the public (reusable) methods of the interface class so one API can be maintained for all code without messing up the use of asynchronous back-ends where an event-loop is available.)

Update: I didn't check the code properly and the first version was completely invalid, hence rewrite.

It actually depends. Foo will be executed synchronously while Bar not. It might happen that the scheduling/distribution might be important, i.e. it might make a difference for final user. — freakish
– freakish, Commented Dec 21, 2015 at 20:04
Yes, foo() is a blocking call. Also run_until_complete cannot be called with already running event loop — Andrew Svetlov
– Andrew Svetlov, Commented Dec 22, 2015 at 11:27

Andrew Svetlov · Accepted Answer · 2015-12-22 12:13:16Z

3

loop.run_until_complete() should be used outside of coroutine on very top level. Calling run_until_complete() with active (running) event loop is forbidden.

Also loop.run_until_complete(f) is a blocking call: execution thread is blocked until f coroutine or future becomes done.

async def is a proper way to write asynchronous code by making concurrent coroutines which may communicate with each other.

async def requires running event loop (either loop.run_until_complete() or loop.run_forever() should be called).

answered Dec 22, 2015 at 12:13

Andrew Svetlov

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2 Comments

Samuel Colvin Over a year ago

Ok, so there's no reasonably way to build an interface which can be used for both synchronous and asynchronous code.

Andrew Svetlov Over a year ago

No. For now the best recommended way is to build async interface and make sync clone of them by adding stub functions running async counterparts with help of run_until_complete()

Jashandeep Sohi · Accepted Answer · 2015-12-21 19:59:11Z

1

There is a world of difference. It is a SyntaxError to use await in a normal function def function.

import asyncio

async def atest():
    print("hello")

def test():
    await atest()

async def main():
    await atest()
    test()

--

File "test.py", line 9
    await atest()
              ^
SyntaxError: invalid syntax

edited Dec 21, 2015 at 19:59

answered Dec 21, 2015 at 19:46

Jashandeep Sohi

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3 Comments

Samuel Colvin Over a year ago

Yes but as explained lets assume we're calling them thing (coroutine or function) from inside a coroutine.

Jashandeep Sohi Over a year ago

No, it's a SyntaxError no mater what. You can easily try it for yourself.

Samuel Colvin Over a year ago

yes you're right, my bad for not checking the code before submitted the question. I'll update the question.

MathMXC · Accepted Answer · 2015-12-21 19:15:22Z

-1

I do not believe so. I have used both and have not really seen a difference. I prefer foo just because I have used it more and understand it a bit better, but it is up to you.

answered Dec 21, 2015 at 19:15

MathMXC

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