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Inheritance

Inheritance offers a mechanism to achieve two aims:

1. Share behaviour/data that is common between classes.

2. Define common interface to allow as yet unknown extensions to be plugged in.

Consider the example of a modelling a university system that contains two types of person: Lecturer and Student.

These two types would share common attributes such as name & age so duplicating these fields in each class is not ideal.

Instead the Person class can be “shared” between the Lecturer and Student classes by setting it as super class:

class Person(object):

    name = None
    age = None

    def __init__(self, name, age):
        self.name = name
        self.age = age

#-----------------------------------

class Student(Person): # Person becomes a super class of Student

    lectures = None

    def __init__(self, name, age, lectures):
        # Basically calls Person.__init__(self, name, age)
        super(Student, self).__init__(name, age)
        self.lectures = lectures

#-----------------------------------

class Lecturer(Person):

    salary = None

    def __init__(self, name, age, salary):
        super(Lecturer, self).__init__(name, age)
        self.salary = salary

#-----------------------------------

# Demo
person1 = Student('Bob', 20, ['science', 'maths'])
person2 = Lecturer('Alice', 40, 28000)

# No parentheses required if accessing attribute
print('Student name: ' + person1.name)
print( 'Lecturer name: ' + person2.name)

print('Student lectures: ' + str(person1.lectures))
#print('Lecturer lectures: ' + str(person2.lectures)) => causes AttributeError

print('Lecturer salary: ' + str(person2.salary))
#print('Student salary: ' + str(person1.salary)) => causes AttributeError