Python unittest – setUpClass() is giving me trouble – why can’t I inherit like this?

setUpClass must be a class method. From the documentation:

A class method called before tests in an individual class run. setUpClass is called with the class as the only argument and must be decorated as a classmethod():

@classmethod
def setUpClass(cls):
    ...

See Class and Module Fixtures for more details.

Your version is missing the @classmethod decorator:

class MyUnitTest(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        do_something_expensive_for_all_sets_of_tests()

class MyFirstSetOfTests(MyUnitTest):
    @classmethod
    def setUpClass(cls):
        super(MyFirstSetOfTests, cls).setUpClass()
        do_something_expensive_for_just_these_first_tests()

The error is thrown because MyFirstSetOfTests.setUpClass() is called on the class, not on an instance, but you didn’t mark your method as a classmethod and thus it was not passed in the automatic self argument. In the above updated code I used cls instead, to reflect that the name references the class object.