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Introduction to Python Programming

13.5 Multiple inheritance and mixin classes

Introduction to Python Programming13.5 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).

Checkpoint

Multiple inheritance organization

Concepts in Practice

Implementing multiple inheritance

Consider the program:

    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)
    
1.
What is the value of c_inst.a_attr?
  1. 1
  2. 5
  3. Error
2.
What is the value of c_inst.c_attr?
  1. 2
  2. 20
  3. Error
3.
What is the value of b_inst.a_attr?
  1. 2
  2. 4
  3. Error

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.

Concepts in Practice

Creating a mixin class

The following code isn't correct, as not all plants are carnivorous. Follow the programmer through the programmer's edits to improve the program. Note: Rose, Pitcher, and VenusFlyTrap represent plants that all photosynthesize. Pitcher and VenusFlyTrap represent plants that are also carnivorous and can eat.

A pass statement is used in Python to indicate that code is to be written later and prevents certain errors that would result if no code was written or a comment was used instead.

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

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

    class Rose(Plant):
      pass

    class Pitcher(Plant):
      pass
    
    class VenusFlyTrap(Plant):
      pass
    
4.
Which edit is appropriate?
  1. Move eat() to a different class.
  2. Remove Plant as a superclass.
5.
The programmer edits the code as follows. How can the program be improved?
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")
  1. Move photosynth() into Rose.
  2. Remove redundancy of eat().
6.
The programmer edits the code to create a mixin class containing eat(). Which class name, replacing X, is most appropriate for the mixin?
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
  1. CarnivClass
  2. CarnivMixin

Try It

Fixing the diamond problem with a mixin class

The program below represents multiple inheritance and has the diamond problem. Edit the program to use a mixin class, OnlineMixin. OnlineMixin:

  • Replaces OnlineShop.
  • Is a superclass of FoodFast.
  • Has one method, process_online(), which prints "Connecting to server" and is called by FoodFast's process_order().

The output should match:

    Processing meal order
    Connecting to server
    Enjoy your FoodFast order
    
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