13.4: Hierarchical Inheritance

Learning Objectives

By the end of this section you should be able to

• Label relationships between classes as types of inheritance.
• Construct classes that form hierarchical inheritance.

Hierarchical inheritance basics

Hierarchical inheritance is a type of inheritance in which multiple classes inherit from a single superclass. Multilevel inheritance is a type of inheritance in which a subclass becomes the superclass for another class. Combining hierarchical and multilevel inheritance creates a tree-like organization of classes.

Checkpoint: Hierarchical organization and types of inheritance

Concepts in Practice \(\PageIndex{1}\): Hierarchical organization

Which is an example of hierarchical inheritance?

1. Class B inherits from Class A
   Class C inherits from Class A
2. Class B inherits from Class A
   Class C inherits from Class B
3. Class B inherits from Class A
   Class C inherits from Class D

Answer

a. Class A is the superclass for Class B and Class C.

Concepts in Practice \(\PageIndex{2}\): Hierarchical organization

Which group of classes is hierarchical inheritance appropriate for?

1. Cat, Dog, Bird
2. Employee, Developer, SalesRep
3. Dessert, BakedGood, ApplePie

Answer

b. Employee would be an appropriate superclass for Developer and SalesRep. Developer and SalesRep are not directly related.

Implementing hierarchical inheritance

Multiple classes can inherit from a single class by simply including the superclass name in each subclass definition.

Example 13.4: Choir members

    class ChoirMember:
      def display(self):
        print("Current choir member")

    class Soprano(ChoirMember):
      def display(self):
        super().display()
        print("Part: Soprano")

    class Soprano1(Soprano):
      def display(self):
        super().display()
        print("Division: Soprano 1")

    class Alto(ChoirMember):
      def display(self):
        super().display()
        print("Part: Alto")

    class Tenor(ChoirMember):
      def display(self):
        super().display()
        print("Part: Tenor")

    class Bass(ChoirMember):
      def display(self):
        super().display()
        print("Part: Bass")

    mem_10 = Alto()
    mem_13 = Tenor()
    mem_15 = Soprano1()

    mem_10.display()
    print()
    mem_13.display()
    print()
    mem_15.display()

The code's output is:

    Current choir member
    Part: Alto

    Current choir member
    Part: Tenor

    Current choir member
    Part: Soprano
    Division: Soprano 1

Concepts in Practice: Implementing hierarchical inheritance

Consider the program:

    class A:
      def __init__(self, a_attr=0):
        self.a_attr = a_attr

    class B(A):
      def __init__(self, a_attr=0, b_attr=0):
        super().__init__(a_attr)
        self.b_attr = b_attr

    class C(A):
      def __init__(self, a_attr=0, c_attr=0):
        super().__init__(a_attr)
        self.c_attr = c_attr

    class D(B):
      def __init__(self, a_attr=0, b_attr=0, d_attr=0):
        super().__init__(a_attr, b_attr)
        self.d_attr = d_attr
    
    b_inst = B(2)
    c_inst = C(c_attr=4)
    d_inst = D(6, 7)

Concepts in Practice \(\PageIndex{3}\): Implementing hierarchical inheritance

What is the value of b_inst.b_attr?

1. 0
2. 2
3. Error

Answer

a. B(2) resolves to B(2, 0), as b_attr has a default value of 0 . a_attr is assigned with 2 .

Concepts in Practice \(\PageIndex{4}\): Implementing hierarchical inheritance

Which attributes does c_inst have access to?

1. a_attr
2. a_attr, c_attr
3. c_attr

Answer

b. c_inst has access to C's instance attribute, c_attr, as well the inherited attribute from A, a_attr.

Concepts in Practice \(\PageIndex{5}\): Implementing hierarchical inheritance

Which attributes does d_inst have access to?

1. b_attr, d_attr
2. a_attr, b_attr, d_attr
3. d_attr

Answer

b. D, which inherits from B, so d_inst inherits b_attr. B also inherits from A, so d_inst inherits d_attr.

Try It: Overriding methods

Define three classes: Instrument, Woodwind, and String.

• Instrument has instance attribute owner, with default value of "unknown".
• Woodwind inherits from Instrument and has instance attribute material with default value of "wood".
• String inherits from Instrument and has instance attribute num_strings, with default value of 4.

The output should match:

    This flute belongs to unknown and is made of silver
    This cello belongs to Bea and has 4 strings

Interactive Code

# Define Instrument class

# Define Woodwind class

# Define String class

print(f"This flute belongs to {flute_1.owner} and is made of {flute_1.material}")
print(f"This cello belongs to {cello_1.owner} and has {cello_1.num_strings} strings")

Answer

class Instrument():
def __init__(self, name='unknown'):
self.owner = name

class Woodwind(Instrument):
def __init__(self, name='unknown', material='wood'):
super().__init__(name)
self.material = material

class String(Instrument):
def __init__(self, name='unknown', num_strings=4):
super().__init__(name)
self.num_strings = num_strings

flute_1 = Woodwind(material='silver')
cello_1 = String('Bea')

print(f"This flute belongs to {flute_1.owner} and is made of {flute_1.material}")
print(f"This cello belongs to {cello_1.owner} and has {cello_1.num_strings} strings")