Question

I’ve got a set of actions which share some base checks. My code looks like this:

def is_valid(param): …some pretty complex things unit-tested on their own…

class BaseAction(object):
    def run(self, param):
        if not is_valid(param):
            raise ValueError(…)

        return self.do_run(param)

class Jump(BaseAction):
    def do_run(self, param): …

class Sing(BaseAction):
    def do_run(self, param): …

How should I unit-test the fact that BaseAction.run performs validation?

I think I could unit-test Jump.run and Sing.run, but then to do it properly I’d need to write tests for each subclass of BaseAction, potentially quite a lot of cases. Also this means coupling test for subclasses with a test for base class’ method.

Answer 1

You would unit-test the is_valid() function itself. You then only have to test for the fact that .run() raises a ValueError exception when passed invalid input:

with self.assertRaises(ValueError):
    objectundertest.run(invalid_parameter)

Since you already have a unittest for is_valid() itself, your unittests for .run() do not need to focus on it’s functionality. You can focus on the functionality unique to .run().

In your case BaseAction is a unit providing a service to subclasses; I’d test this as a mock subclass:

class MockAction(BaseAction):
    run_called = None

    def do_run(self, param):
        self.run_called = param

so you can check if run indeed called do_run with the expected param value.