inherit.py

class Parent(object):
    def __init__(self):
        # I inherit from object, so I don't need to call:
        # super().__init__()
        # right? If Python only had single inheritance, that would be
        # true. I could see, statically, what method is next in the
        # lookup path.
        pass


class Child(Parent):
    pass


class FooMixin(object):
    def __init__(self):
        # Because Parent.__init__ forgets to call super(), this method
        # never gets called.
        self.x = 0

    def foo(self):
        # Since __init__ never got run, this will error.
        self.x += 1


class SurprisinglyThisDoesntWork(Child, FooMixin):
    pass


print("The method lookup ordering for SurprisinglyThisDoesntWork is:")
print(SurprisinglyThisDoesntWork.mro())

instance = SurprisinglyThisDoesntWork()
instance.foo() # boom!

# Output of running this script:
#
# $ python3 inherit.py
# The method lookup ordering for SurprisinglyThisDoesntWork is:
# [<class '__main__.SurprisinglyThisDoesntWork'>, <class '__main__.Child'>, <class '__main__.Parent'>, <class '__main__.FooMixin'>, <class 'object'>]
# Traceback (most recent call last):
#   File "inherit.py", line 34, in <module>
#     instance.foo() # boom!
#   File "inherit.py", line 23, in foo
#     self.x += 1
# AttributeError: 'SurprisinglyThisDoesntWork' object has no attribute 'x'