Multiple Inheritance

Classes can inherit from more than one class at the same time, which we called multiple inheritance.

Syntax

To inherit from multiple classes, reference them in a comma separated list in the class definition

class ParentClassName(ParentClass1Name, ParentClass2Name, ..., ParentClassNName):

Note

Check the next page for an example!

Method Resolution Order

With multiple inheritance, parent classes may or may not implement any given method. Therefore the Python interpreter needs to follow a principled procedure to select which method from any number of parent classes to use.

This is defined by the Method Resolution Order (MRO), which to summarise works as follows:

1. The parent classes are searched from left to right as defined and followed up the chain.

2. However if two inherited classes are on the same “level” in an inheritance chain they are searched in the left to right order as defined.

If you’re interested in learning more, follow this link https://docs.python.org/3/howto/mro.html#examples

super()

Due to the MRO, super() will refer to the leftmost parent class. However in many cases you would need to also properly initialise attributes from more than one parent class.

To achieve this you will need to manually call __init__ on each parent class.

Example

class A:
    def __init__(self):
        print("A")

class B:
    def __init__(self):
        print("B")

class C(A, B):
    def __init__(self):
        A.__init__(self)
        B.__init__(self)
        print("C")

instance = C()

Mixins

A mixin class is a way of adding functionality through multiple inheritance. Mixin classes are typically independent from other classes.

Mixins can be used to simplify your code as terminating child classes can combine multiple mixins to achieve their desired functionality. This also increases code resuse - functionality only needs to be written in a single mixin and can be reused by many classes.

Example

Note

The next page includes a complete working example of a mixin.

In the example below classes A and B are mixins that add methods cool_method and awesome_method respectively.

We can then pick and choose from A and B to add functionality from classes e.g.

• class C uses both A and B

• class D uses only B

class A:
    def cool_method(self):
        print("Cool method")

class B:
    def aweseome_method(self, x):
        print(x * 10)

class C(A, B):
    def __init__(self):
        print("C")

class D(B):
    def __init__(self):
        print("D")

c = C()
c.cool_method()
c.aweseome_method(3)

d = D()
d.awesome_method(10)