Python asyncio - can I call a coroutine from a function?

Question

I am working with a non asyncio library that does not do i/o, only simple calculations. One of its features is a series of callbacks on a set of user provided observers in the order they happen, while the user controls a higher level api.

class Calculator:
    def __init__(self):
        self._internal = 0
        self._observers = [

    def register(observer):
        self._observers.append(observer)

    def add(int val):
        for i in range(val):
            self._internal += 1

    def _call_observers(self):
        for o in self._observers:
            o.on_add()

    @property
    def value(self):
        return self._internal

I need to use this from a couroutine to call other coroutines. One idea I had was to subclass an AsyncCalculator and override relevant methods to be coroutines, storing a reference to the event loop.

(not real code)

class AsyncCalculator(Calculator):
    def __init__(self, loop):
        ...

    async def _call_observers(self):
        ...

I was wondering if there is a way to wait on a coroutine from a normal function if there is a reference to the event loop or any other structure available? It seems that if the function is being called from a coroutine, then there might be some hacky way to await another coroutine from the function. Then maybe that hack could be thrown in a utility?

user4815162342 user4815162342 · Accepted Answer · 2018-09-11T13:03:35

If an Calculator method is being invoked from a running event loop, you can instruct a coroutine to resume from a synchronous function. (Asyncio does such things internally all the time.) To do that, you don't need to make _call_observers async, you can do something like this:

class _Observer:
    def __init__(self, fut):
        self.fut = fut
    def on_add(self):
        self.fut.set_result('add')

async def my_coro(calc):
    loop = asyncio.get_event_loop()
    fut = loop.create_future()
    calc.register(_Observer(fut))
    # wait for the observer to be invoked
    op = await fut
    if op == 'add':
        ...

This requires the whole thing to be running inside the event loop, regardless of whether IO is occurring.

Python asyncio - can I call a coroutine from a function?

1 Answers