PythonInheritance
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 namedPerson, withfirstname andlastname properties, and aprintname method:
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()
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 namedStudent, which will inherit the properties and methods from thePerson class:
pass
Note: Use thepass 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 theStudent class to create an object, and then execute theprintname method:
x.printname()
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 thepass 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 theStudent class:
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__() functionoverrides 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
def __init__(self, fname, lname):
Person.__init__(self, fname, lname)
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 asuper() function that will make the child class inherit all the methods and properties from its parent:
Example
def __init__(self, fname, lname):
super().__init__(fname, lname)
By using thesuper() 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 calledgraduationyear to theStudent class:
def __init__(self, fname, lname):
super().__init__(fname, lname)
self.graduationyear = 2019
In the example below, the year2019 should be a variable, and passed into theStudent class when creating student objects.To do so, add another parameter in the__init__() function:
Example
Add ayear parameter, and pass the correct year when creating objects:
def __init__(self, fname, lname, year):
super().__init__(fname, lname)
self.graduationyear = year
x = Student("Mike", "Olsen", 2019)
Add Methods
Example
Add a method calledwelcome to theStudent class:
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)
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.
