Mocking async call in python 3.5

Question

How do I mock async call from one native coroutine to other one using unittest.mock.patch?

I currently have quite an awkward solution:

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Answer 1

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