[TIP] Unit testing functions which call super

Eric Larson eric at ionrock.org
Wed Aug 13 14:09:16 PDT 2014

Marcin Tustin writes:

> Hi All,
> I've come across a method which I needed to add a unit test for, which calls super, e.g.
> def foofunc(self):
>     x= do_some_stuff()
>     if x:
>         return x
>     return super(WhatevClass, self).foofunc()
> Now, I don't want to integration test the whole stack of super foofunc methods, just unit test that one, down to the super call.
> I've accomplished this with, in my test (excuse the 8 leading spaces):
> def setUp(self):
>         self.method_archive = {}
>         for klass in WhatevClass.__mro__[1:]:
>             # we use the class as its own sentinel
>             meth_or_klass = klass.__dict__.get('foofunc', klass)
>             if meth_or_klass != klass:
>                 self.method_archive[klass] = meth_or_klass
>                 del klass.foofunc
>         # now patch an arbitrary member of the mro to supply a dummy foofunc
>         self.sentinel_klass = next(iter(self.method_archive))
>         self.mock_sentinel = mock.MagicMock()
>         self.sentinel_klass.foofunc = self.mock_sentinel
> And corresponding unpatching in tearDown.
> Is there any other approach which is standard, or objectively better?

What about patching super?

    from mock import patch

    class Parent(object):

        def foo(self):
            print('foo from parent')

    class Kid(Parent):

        def foo(self):
            self.called_foo = True
            super(Kid, self).foo()

    class TestKid(object):

        @patch('%s.super' % __name__, create=True)
        def test_foo(self, super_func):
            kid = Kid()
            assert kid.called_foo
            assert super_func.called

That does the unpatching for you and hopefully reduces the potential of
the patch leaking into other tests.

The downside is that this wouldn't cover the use case where an explicit
parent method was called. Ie:

    def foo(self):
        return Parent.foo()

It sounds like if you are using multiple inheritance you're already avoiding
this pattern.



> Thanks,
> Marcin
> Marcin Tustin
> Senior Software Engineer
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