Mocking a function with different return values in Python

For the first half of 2019 I was borrowed by the Performance Team at Mozilla and asked to help them implement a way to measure the performance of various browsers on Android platforms. This has involved me getting familiar with several tools and environments that I had not touched before:

  • Mozilla's tooling for use with mozilla-central
  • Rooting Android devices for testing purposes
  • Android Studio and it's device emulation tools
  • Executing and parsing shell commands

Of course, I am using pytest as well to make sure that I had thought about testing scenarios and giving whomever supports this code once I am back at my previous role a chance to figure out what is going on.

One of the testing scenarios I came up with was handing the fact that I had designed the code to have a fallback mode in case the shell command I was using did not work on that particular device. Some code reviews from other developers revealed some differences in how the top command works on different phones.

So, drawing on my own experiences with using test doubles in PHP, I asked myself "how can I create a mock that has different return values?". In pytest this functionality is called side_effect.

So, here is some of the code in question that I needed to test:

    cpuinfo = raptor.device.shell_output("top -O %CPU -n 1").split("\n")
    raptor.device._verbose = verbose
    for line in cpuinfo:
    data = line.split()
            if data[-1] == app_name:
                cpu_usage = data[3]
except Exception:
    cpuinfo = raptor.device.shell_output("dumpsys cpuinfo | grep %s" % app_name).split("\n")
    for line in cpuinfo:
        data = line.split()
        cpu_usage = data[0].strip('%')

I wrapped the first call in a try-catch construct because an exception is thrown if anything is up with that top call. If that call doesn't work, I then want to use that dumpsys call.

In the test itself, I would need the shell_output call to first throw an exception (as expected for the scenario) and then return some output that I can then parse and use.

In the PHP world, most test doubling tools allow you to create a mock and have it return different values on consecutive calls. Pytest is no different, but it took me a while to figure out the correct search terms to find the functionality I wanted.

So, here is how I did it:

def test_usage_with_fallback():
    with mock.patch('mozdevice.adb.ADBDevice') as device:
        with mock.patch('raptor.raptor.RaptorControlServer') as control_server:
            First we have to return an error for 'top'
            Then we return what we would get for 'dumpsys'
            device.shell_output.side_effect = [
                OSError('top failed, move on'),
                ' 34% 14781/org.mozilla.geckoview_example: 26% user + 7.5% kernel'
            device._verbose = True

            # Create a control server
            control_server.cpu_test = True
            control_server.test_name = 'cpuunittest'
            control_server.device = device
            control_server.app_name = 'org.mozilla.geckoview_example'
            raptor = Raptor('geckoview', 'org.mozilla.geckoview_example', cpu_test=True)
            raptor.device = device
            raptor.config['cpu_test'] = True
            raptor.control_server = control_server

            # Verify the response contains our expected CPU % of 34
            cpuinfo_data = {
                u'type': u'cpu',
                u'test': u'usage_with_fallback',
                u'unit': u'%',
                u'values': {
                    u'browser_cpu_usage': '34'

Let me break down what I did (as always, I am open to suggestions on better ways to write this test).

The first double is for a class that communicates with the Android device. Then the next double I needed was for the "control server", which is what is used to control the browser and execute tests.

My first "side effect" is to generate an error so it triggers the first condition of the scenario that 'top should not work'. The second "side effect" is the response I am expecting to get from the shell command in my fallback area of the code.

Then I continue with the "arrange" part of the Arrange-Act-Assert testing pattern I like to use -- I configure my "control server" to be the way I want it.

I finish up with creating what I expect the data that is to be submitted to our internal systems looks like.

I execute the code I am testing (the "act" part) and then use a spy to make sure the control server would have submitted the data I was expecting to have been generated.

The ability to have a method return different values is powerful in the context of writing tests for code that has conditional behaviour. I hope you find this example useful!