Mocking Asynchronous Functions In Python
You might have already heard about Python’s
asyncio module, it allows you to easily run concurrent
code using Python.
In the last few months I’ve worked in some codebases which take advantage of the benefits of
asyncio, mainly through their use of aiohttp.
asyncio you’ll use the
async/await keywords to both define and call
This also means changes in the way you test your code because, unlike ordinary functions,
asynchronous functions always return a coroutine object, which needs to be awaited, using the
await keyword in order to actually schedule it, run it and get the actual return value.
What we’re mocking
In this example, we’re going to be mocking a simple function which adds two integers, its parameters,
while resorting to
asyncio.sleep to simulate IO heavy tasks, for example, HTTP requests or a
import asyncio async def sum(x, y): await asyncio.sleep(1) return x + y
Asynchronous functions in Python return what’s known as a
Future object, which contains the
result of calling the asynchronous function.
As such, the “secret” to mocking these functions is to make the patched function return a
Future object with the result we’re expecting, as one can see in the example below.
import pytest import asyncio @pytest.fixture() def mock_sum(mocker): future = asyncio.Future() future.set_result(4) mocker.patch('app.sum', return_value=future)
As you can see in the example above, we’re creating a pytest fixture, namely
patches the function we created at the beginning of the post and specifies that the function call
will return a
Future object, with a result of
In your own tests you will, of course, need to change the call to
set_result to return whatever
value you’re expecting, maybe a HTTP response or some database query result.
With this done we can now create a simple test case that tests the
import pytest import asyncio @pytest.mark.asyncio async def test_sum(mock_sum): result = await sum(1, 2) # I know 1+2 is equal to 3 but one man can only dream! assert result == 4
There’s also a few different things happening here when compared to a regular test function:
@pytest.mark.asynciodecorator - This tells pytest that this is an asynchronous test function, otherwise pytest will skip it.
async def test_sum(mock_sum)- Defines the asynchronous test function while at the same time calls the pytest fixture,
mock_sum, so that it successfully mocks the
result = await sum(1,2)- Correctly calls the asynchronous function using the
1 + 2 is equal to
3 I’m purposefully asserting that this returns
4 so as to make sure
that the fixture is indeed called.
If you go ahead and run
pytest now with the code shown above you should see that indeed it
executes successfully, passing the tests.
However, imagine that you want to mock the
sum function multiple times while having a different
value provided to
set_result in the
Future object. It doesn’t make sense to create multiple
fixtures since we’ll be repeatedly patching the same function. In this case we’ll return the
Future object and call the
set_result function in the test function, thus,
our fixture we’ll now look like:
import pytest import asyncio @pytest.fixture() def mock_sum(mocker): future = asyncio.Future() mocker.patch('app.sum', return_value=future) return future
Notice that we’re not calling
set_result in the fixture this time around. With the updated
fixture we now need to update the test function to look like this:
import pytest import app @pytest.mark.asyncio async def test_sum(mock_sum): mock_sum.set_result(4) result = await app.sum(1, 2) # I know 1+2 is equal to 3 but one man can only dream! assert result == 4
Finally, notice now how we’re calling
mock_sum.set_result(4). If we want the mock to return
different values we now just need to change the value provided to
set_result instead of having to
create multiple fixture for different tests!
Mocking It In Python 3.8
The code above only works for versions of Python <3.8. In Python 3.8 we need to change the code
With that said, we can simply change the mocking function to return the
AsyncMock instance instead
from unittest.mock import AsyncMock @pytest.fixture() def mock_sum(mocker): async_mock = AsyncMock(return_value=4) mocker.patch('app.sum', side_effect=async_mock)
As you can see in the code above, the main change is that the return value is now set as an
AsyncMock instance instead of a
Future instance, and we can also now use the
argument in the
AsyncMock instantiation instead of needing to call a function afterwards to set
With the code above our test function will look like the first showed in this blog post, where we
don’t change the result of the mock. However, as we did in the end of the previous section, if we
need to mock the same function multiple times while having different results it’s better if we
just return the
AsyncMock instance from the fixture and set the
return_value in the test
function. As such, our fixture would now look like this:
from unittest.mock import AsyncMock @pytest.fixture() def mock_sum(mocker): async_mock = AsyncMock() mocker.patch('app.sum', side_effect=async_mock) return async_mock
And with this fixture we could simply update our test function to the following:
@pytest.mark.asyncio async def test_sum(mock_sum): mock_sum.return_value = 4 result = await app.sum(1, 2) assert result == 4
Notice that the only change compared to the previous section is that we now set the
attribute of the mock instead of calling the
set_result function seeing as we’re now working with
AsyncMock instead of
Future. Aside that, the test function looks exactly the same.
In conclusion mocking asynchronous functions in Python is actually easier than I expected at first, mostly because I didn’t really understood how asyncio worked. After some reading and experimentation it turns out it’s quick and easy to do, and it allows you to run concurrent tests, which should speed up your test suite!
If you’re reading this for a quick solution and don’t really known what’s going on I’d advise reading up on asyncio.
I hope this blogpost has helped you! 👋