Python Inheritance

Summary: in this tutorial, you’ll learn about Python inheritance and how to use the inheritance to reuse code from an existing class.

Introduction to the Python inheritance

Inheritance allows a class to reuse the logic of an existing class. Suppose you have the following Person class:

class Person: def __init__(self, name): self.name = name def greet(self): return f"Hi, it's {self.name}"
Code language: Python (python)

The Person class has the name attribute and the greet() method.

Now, you want to define the Employee that is similar to the Person class:

class Employee: def __init__(self, name, job_title): self.name = name self.job_title = job_title def greet(self): return f"Hi, it's {self.name}"
Code language: Python (python)

The Employee class has two attributes name and job_title. It also has the greet() method that is exactly the same as the greet() method of the Person class.

To reuse the greet() method from the Person class in the Employee class, you can create a relationship between the Person and Employee classes. To do it, you use inheritance so that the Employee class inherits from the Person class.

The following redefines the Employee class that inherits from the Person class:

class Employee(Person): def __init__(self, name, job_title): self.name = name self.job_title = job_title
Code language: Python (python)

By doing this, the Employee class behaves the same as the Person class without redefining the greet() method.

This is a single inheritance because the Employee inherits from a single class (Person). Note that Python also supports multiple inheritances where a class inherits from multiple classes.

Since the Employee inherits attributes and methods of the Person class, you can use the instance of the Employee class as if it were an instance of the Person class.

For example, the following creates a new instance of the Employee class and call the greet() method:

employee = Employee('John', 'Python Developer') print(employee.greet())
Code language: Python (python)

Output:

Hi, it's John
Code language: Python (python)

Inheritance terminology

The Person class is the parent class, the base class, or the super class of the Employee class. And the Employee class is a child class, a derived class, or a subclass of the Person class.

The Employee class derives from, extends, or subclasses the Person class.

The relationship between the Employee class and Person class is IS-A relationship. In other words, an employee is a person.

type vs. isinstance

The following shows the type of instances of the Person and Employee classes:

person = Person('Jane') print(type(person)) employee = Employee('John', 'Python Developer') print(type(employee))
Code language: Python (python)

Output:

<class '__main__.Person'> <class '__main__.Employee'>
Code language: Python (python)

To check if an object is an instance of a class, you use the isinstance() method. For example:

person = Person('Jane') print(isinstance(person, Person)) # True employee = Employee('John', 'Python Developer') print(isinstance(employee, Person)) # True print(isinstance(employee, Employee)) # True print(isinstance(person, Employee)) # False
Code language: Python (python)

Output:

True True True False
Code language: Python (python)

As clearly shown in the output:

  • The person is an instance of the Person class.
  • The employee is an instance of the Employee class. It’s also an instance of the Person class.
  • The person is not an instance of the Employee class.

In practice, you’ll often use the isinstance() function to check whether an object has certain methods. Then, you’ll call the methods of that object.

issubclass

To check if a class is a subclass of another class, you use the issubclass() function. For example:

print(issubclass(Employee, Person)) # True
Code language: Python (python)

The following defines the SalesEmployee class that inherits from the Employee class:

class SalesEmployee(Employee): pass
Code language: Python (python)

The SalesEmployee is the subclass of the Employee class. It’s also a subclass of the Person class as shown in the following:

print(issubclass(SalesEmployee, Employee)) # True print(issubclass(SalesEmployee, Person)) # True
Code language: Python (python)

Note that when you define a class that doesn’t inherit from any class, it’ll implicitly inherit from the built-in object class.

For example, the Person class inherits from the object class implicitly. Therefore, it is a subclass of the object class:

print(issubclass(Person, object)) # True
Code language: Python (python)

In other words, all classes are subclasses of the object class.

Summary

  • Inheritance allows a class to reuse existing attributes and methods of another class.
  • The class that inherits from another class is called a child class, a subclass, or a derived class.
  • The class from which other classes inherit is call a parent class, a super class, or a base class.
  • Use isinstance() to check if an object is an instance of a class.
  • Use issubclass() to check if a class is a subclass of another class.
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