Python Inheritance

Python Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

Parent class is the class being inherited from, also called base class.

Child class is the class that inherits from another class, also called derived class.

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Create a Parent Class

Any class can be a parent class, so the syntax is the same as creating any other class:

Example

Create a class named Person, with firstname and lastname properties, and a printname method:

class Person:
  def __init__(self, fname, lname):
    self.firstname = fname
    self.lastname = lname

  def printname(self):
    print(self.firstname, self.lastname)

#Use the Person class to create an object, and then execute the printname method:
x = Person("John", "Doe")
x.printname()

Show Python »
C:\Users\My Name>python demo_inheritance_parent.py
John Doe                                          
                                                  
                                                  

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Create a Child Class

To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:

Example

Create a class named Student, which will inherit the properties and methods from the Person class:

class Student(Person):
  pass

Note: Use the pass keyword when you do not want to add any other properties or methods to the class.

Now the Student class has the same properties and methods as the Person class.

Example

Use the Student class to create an object, and then execute the printname method:

x = Student("Mike", "Olsen")
x.printname()

Show Python »
C:\Users\My Name>python demo_inheritance_child.py
Mike Olsen                                       
                                                 
                                                 

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Add the __init__() Function

So far we have created a child class that inherits the properties and methods from its parent.

We want to add the __init__() function to the child class (instead of the pass keyword).

Note: The __init__() function is called automatically every time the class is being used to create a new object.

Example

Add the __init__() function to the Student class:

class Student(Person):
  def __init__(self, fname, lname):
    #add properties etc.

When you add the __init__() function, the child class will no longer inherit the parent's __init__() function.

Note: The child's __init__() function overrides the inheritance of the parent's __init__() function.

To keep the inheritance of the parent's __init__() function, add a call to the parent's __init__() function:

Example

class Student(Person):
  def __init__(self, fname, lname):
    Person.__init__(self, fname, lname)

Show Python »
C:\Users\My Name>python demo_inheritance_init.py
Mike Olsen                                      
                                                
                                                

Now we have successfully added the __init__() function, and kept the inheritance of the parent class, and we are ready to add functionality in the __init__() function.

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Add Properties

Example

Add a property called graduationyear to the Student class:

class Student(Person):
  def __init__(self, fname, lname):
    Person.__init__(self, fname, lname)
    self.graduationyear = 2019

Show Python »
C:\Users\My Name>python demo_inheritance_add_prop.py
2019                                                
                                                    
                                                    

In the example below, the year 2019 should be a variable, and passed into the Student class when creating student objects. To do so, add another parameter in the __init__() function:

Example

Add a year parameter, and pass the correct year when creating objects:

class Student(Person):
  def __init__(self, fname, lname, year):
    Person.__init__(self, fname, lname)
    self.graduationyear = year

x = Student("Mike", "Olsen", 2019)

Show Python »
C:\Users\My Name>python demo_inheritance_add_prop2.py
2019                                                 
                                                     
                                                     

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Add Methods

Example

Add a method called welcome to the Student class:

class Student(Person):
  def __init__(self, fname, lname, year):
    Person.__init__(self, fname, lname)
    self.graduationyear = year

  def welcome(self):
    print("Welcome", self.firstname, self.lastname, "to the class of", self.graduationyear)

Show Python »
C:\Users\My Name>python demo_inheritance_add_method.py
Welcome Mike Olsen to the class of 2019               
                                                      
                                                      

If you add a method in the child class with the same name as a function in the parent class, the inheritance of the parent method will be overridden.