Python supports a simple type of multiple inheritance which allows the creation of Mixins. Mixins are a sort of class that is used to “mix in” extra properties and methods into a class. This allows you to create classes in a compositional style.

Mixins are a really great concept but I often find that people use them incorrectly which can lead to some bugs. I often see Mixins used like the following:

class Mixin1(object):
    def test(self):
        print "Mixin1"

class Mixin2(object):
    def test(self):
        print "Mixin2"

class MyClass(BaseClass, Mixin1, Mixin2):
    pass

However, in Python the class hierarchy is defined right to left, so in this case the Mixin2 class is the base class, extended by Mixin1 and finally by BaseClass. This is usually fine because many times the mixin classes don’t override each other’s, or the base class’ methods. But if you do override methods or properties in your mixins this can lead to unexpected results because the priority of how methods are resolved is from left to right.

>>> obj = MyClass()
>>> obj.test()
Mixin1

The correct way to use mixins is like in the reverse order:

class MyClass(Mixin2, Mixin1, BaseClass):
    pass

This kind of looks counter-intuitive at first because most people would read a top-down class hierarchy from left to right but if you include the class you are defining, you can read correctly up the class hierarchy (MyClass => Mixin2 => Mixin1 => BaseClass. If you define your classes this way you won’t have to many conflicts and run into too many bugs.

>>> obj = MyClass()
>>> obj.test()
Mixin2