Multiple inheritance - How to call method_x in InheritedBaseB from method_x in InheritedBaseA?

The Music Guy musicguy at alphaios.net
Fri Sep 4 12:01:48 CEST 2009


I have a peculiar problem that involves multiple inheritance and method calling.

I have a bunch of classes, one of which is called MyMixin and doesn't
inherit from anything. MyMixin expects that it will be inherited along
with one of several other classes that each define certain
functionality. It defines method_x, which it assumes will also be
defined in the other class that MyMixin ends up getting inherited
with. For example,

class MyMixin(object):
    def method_x(self, a, b, c):
        ...

class BaseA(object):
    def method_x(self, a, b, c):
        ...

class BaseB(object):
    def method_x(self, a, b, c):
        ...

class BaseC(object):
    def method_x(self, a, b, c):
        ...

class FooX(MyMixin, BaseA):
    ...

class FooY(MyMxin, BaseB):
    ...

class FooZ(MyMixin, BaseC):
    ...


This all appears fine at first, but there is a problem: Each Foo's
method_x must call the method_x of MyMixin as well as the method_x of
each respective Foo's second base class. One cannot simply call
FooN.method_x, because that will only call MyMixin.method_x and not
that of the other base.

One might be tempted to amend MyMixin's method_x so that it calls the
parent's method_x before doing anything else:

class MyMixin(object):
    def method_x(self, a, b, c):
        super(MyMixin, self).method_x(a, b, c)
        ...

...but of course, that will fail with an AttributeError because
MyMixin's only superclass is object, which does not have a method_x.

The only way I can think to solve the problem would be to implement a
method_x for each Foo that calls the method_x for each of the bases:

class FooX(MyMixin, BaseA):
    def method_x(self, a, b, c):
        MyMixin.method_x(self, a, b, c)
        BaseA.method_x(self, a, b, c)

class FooY(MyMxin, BaseB):
    def method_x(self, a, b, c):
        MyMixin.method_x(self, a, b, c)
        BaseB.method_x(self, a, b, c)

class FooZ(MyMixin, BaseC):
    def method_x(self, a, b, c):
        MyMixin.method_x(self, a, b, c)
        BaseC.method_x(self, a, b, c)


The problem with this solution is that method_x has to be explicitly
created for each Foo, even though they all do just about the same
thing, which kind of defeats the purpose of using multiple inheritance
in this situation. Besides that, I just don't like it!

So, does anyone have an idea about how to remedy this, or at least
work around it?



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