Mocking async call in python 3.5

Everyone's missing what's probably the simplest and clearest solution:

@patch('some.path')
def test(self, mock):
    f = asyncio.Future()
    f.set_result('whatever result you want')
    process_smtp_message.return_value = f
    mock.assert_called_with(1, 2, 3)

remember a coroutine can be thought of as just a function which is guaranteed to return a future which can, in turn be awaited.

The solution was actually quite simple: I just needed to convert __call__ method of mock into coroutine:

class AsyncMock(MagicMock):
    async def __call__(self, *args, **kwargs):
        return super(AsyncMock, self).__call__(*args, **kwargs)

This works perfectly, when mock is called, code receives native coroutine

Example usage:

@mock.patch('my.path.asyncio.sleep', new_callable=AsyncMock)
def test_stuff(sleep):
    # code

Subclassing MagicMock will propagate your custom class for all the mocks generated from your coroutine mock. For instance, AsyncMock().__str__ will also become an AsyncMock which is probably not what you're looking for.

Instead, you might want to define a factory that creates a Mock (or a MagicMock) with custom arguments, for instance side_effect=coroutine(coro). Also, it might be a good idea to separate the coroutine function from the coroutine (as explained in the documentation).

Here is what I came up with:

from asyncio import coroutine

def CoroMock():
    coro = Mock(name="CoroutineResult")
    corofunc = Mock(name="CoroutineFunction", side_effect=coroutine(coro))
    corofunc.coro = coro
    return corofunc

An explanation of the different objects:

• corofunc: the coroutine function mock
• corofunc.side_effect(): the coroutine, generated for each call
• corofunc.coro: the mock used by the coroutine to get the result
• corofunc.coro.return_value: the value returned by the coroutine
• corofunc.coro.side_effect: might be used to raise an exception

Example:

async def coro(a, b):
    return await sleep(1, result=a+b)

def some_action(a, b):
    return get_event_loop().run_until_complete(coro(a, b))

@patch('__main__.coro', new_callable=CoroMock)
def test(corofunc):
    a, b, c = 1, 2, 3
    corofunc.coro.return_value = c
    result = some_action(a, b)
    corofunc.assert_called_with(a, b)
    assert result == c

Based on @scolvin answer I created this (imo) cleaner way:

import asyncio

def async_return(result):
    f = asyncio.Future()
    f.set_result(result)
    return f

That's it, just use it around whatever return you want to be async, as in

mock = MagicMock(return_value=async_return("Example return"))
await mock()