Class inheritance in Python

We distinguish the parent/source/superclass class and the subclass that extends it.

A subclass can have more than one parent class that it extends, but this is atypical.

The subclass will inherit all attributes and methods of the superclass unless it overrides them.

In addition to overriding the methods of the parent we can extend them in the child. This is a bit like using an abstract.

Example of class inheritance


class Person:
    """ Our superclass
    """
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def birthday(self):
        print('Happy birthday you were', self.age)
        self.age += 1
        print('You are now', self.age)

    def __str__(self):
        return self.name + ' is ' + str(self.age)


class Employee(Person):
    """ Our subclass that extends `Person`
    """
    def __init__(self, name, age, id):
        super().__init__(name, age)
        self.id = id

    def calculate_pay(self, hours_worked):
        rate_of_pay = 7.50
        if self.age >= 21:
            rate_of_pay += 2.50
        return hours_worked * rate_of_pay

    def __str__(self):
        return super().__str__() + ' - id(' + str(self.id) + ')'

Key points

  • We pass the name of the parent class to the child class as a parameter
  • We must include all of the parent class’s attributes in the constructor of the subclass
  • We must use a special super().__init__(..,...) method to enact the extension of the parent. This method references the __init__ class in the parent.

Protected/private methods and attributes

Protected methods and attributes are private to the class but can be accessed from any subclass

Private methods cannot be accessed from anywhere outside of the class

We declare private methods and attributes with a double underscore (__). We declare protected methods and attributes with a single underscore (_).

This is convention, there are not actually private and protected attributes or properties.

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