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 thePerson
class. - The
employee
is an instance of theEmployee
class. It’s also an instance of thePerson
class. - The
person
is not an instance of theEmployee
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.