13.5 Multiple inheritance and mixin classes

Learning objectives

By the end of this section you should be able to

• Construct a class that inherits from multiple superclasses.
• Identify the diamond problem within multiple inheritance.
• Construct a mixin class to add functionality to a subclass.

Multiple inheritance basics

Multiple inheritance is a type of inheritance in which one class inherits from multiple classes. A class inherited from multiple classes has all superclasses listed in the class definition inheritance list. Ex: class SubClass(SuperClass_1, SuperClass_2).

    class A:
      def __init__(self, a_attr=2):
        self.a_attr = a_attr

    class B:
      def __init__(self, b_attr=4):
        self.b_attr = b_attr

    class C(A, B):
      def __init__(self, a_attr=5, b_attr=10, c_attr=20):
        A.__init__(self, a_attr)
        B.__init__(self, b_attr)
        self.c_attr = c_attr

    b_inst = B(2)
    c_inst = C(1, 2)
    

The diamond problem and mixin classes

Multiple inheritance should be implemented with care. The diamond problem occurs when a class inherits from multiple classes that share a common superclass. Ex: Dessert and BakedGood both inherit from Food, and ApplePie inherits from Dessert and BakedGood.

Figure 13.2 The diamond problem. If both

Dessert

and

BakedGood

override a

Food

method, the overridden

Food

method that

ApplePie

inherits is ambiguous. Thus, diamond shaped inheritance should be avoided.

Mixin classes promote modularity and can remove the diamond problem. A mixin class:

• Has no superclass
• Has attributes and methods to be added to a subclass
• Isn't instantiated (Ex: Given MyMixin class, my_inst = MyMixin() should never be used.)

Mixin classes are used in multiple inheritance to add functionality to a subclass without adding inheritance concerns.

    class Plant:
      def photosynth(self):
        print("Photosynthesizing")

      def eat(self):
        print("Eating")

    class Rose(Plant):
      pass

    class Pitcher(Plant):
      pass
    
    class VenusFlyTrap(Plant):
      pass
    

class Plant:
  def photosynth(self):
    print("Photosynthesizing")

class Rose(Plant):
  pass

class Pitcher(Plant):
  def eat(self):
    print("Eating")

class VenusFlyTrap(Plant)
  def eat(self):
    print("Eating")

class Plant:
  def photosynth(self):
    print("Photosynthesizing")

class X:
  def eat(self):
    print("Eating")

class Rose(Plant):
  pass

class Pitcher(Plant, X):
  pass

class VenusFlyTrap(Plant, X)
  pass