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.

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()

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Example

Create a class named Person, with

firstname and lastname properties,

and a printname method:

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

Example

Create a class named Student, which will inherit the properties

and methods from

the Person class:

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()

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Example

Use the Student class to create an object,

and then execute the printname method:

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.

Example

Add the __init__() function to the

Student class:

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)

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Example

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.

Use the super() Function

Python also has a super() function that

will make the child class inherit all the methods and properties from its

parent:

Example



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

super().__init__(fname, lname)

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Example

By using the super() function, you do not

have to use the name of the parent element, it will automatically inherit the

methods and properties from its parent.

Add Properties

Example

Add a property called graduationyear to the

Student class:



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

super().__init__(fname, lname)    self.graduationyear

= 2019

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Example

Add a property called graduationyear to the

Student class:

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):   

super().__init__(fname, lname)    self.graduationyear

= yearx = Student("Mike", "Olsen", 2019)

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Example

Add a year parameter, and pass the correct

year when creating objects:

Add Methods

Example

Add a method called welcome to the

Student class:



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

super().__init__(fname, lname)    self.graduationyear

= year  def welcome(self):    print("Welcome",

self.firstname, self.lastname, "to the class of", self.graduationyear)

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Example

Add a method called welcome to the

Student class:

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.

Test Yourself With Exercises

Exercise:

What is the correct syntax to create a class named Student that will inherit properties and methods from a class named Person?

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