test.py

class Base(object):
    def process(self, s=''):
        return s


class M1(object):
    def process(self, *args, **kwargs):
        s = super(M1, self).process(*args, **kwargs)
        s += '_1_'
        return s


class M2(object):
    def process(self, *args, **kwargs):
        s = super(M2, self).process(*args, **kwargs)
        s += '_2_'
        return s


class M3(object):
    def process(self, *args, **kwargs):
        s = super(M3, self).process(*args, **kwargs)
        s += '_3_'
        return s


class App(M3, M2, M1, Base):
    pass


app = App()
print(app.process())