I think one of the best ways of understanding the way inheritance works if you've never used it before is with an example and so that's what we're going to walk through this guide. So I'm going to create a class of user and this is going to be a pretty standard user.

This is going to be an abstract user that simply has three attributes, so I'll say def double underscore init, pass in self, and then we're going to pass in an e-mail, first name, and then a last name. These are all standard attributes so you'd expect in a User class. From there you can create some instance attributes so we'll create one for email, one for first name, and then one for a last name and that's going to be all we really need to do for the attributes.

Now let's add some behavior and so we're going to go with a greeting so I'll say def greeting. It expects self as the only argument and then it's going to return a formatted string so format and say hi and pass in with string interpellation self.first_name and then self.last_name and that's all that we're going to need in this class.

def greeting(self):
    return f'Hi {self.first_name} {self.last_name}'

But imagine that you're building out a full web application and you need to have the concept of admin users so admin users are going to be a specific type of user. And whenever you're trying to decide if you want to use inheritance you can always ask yourself is this new element that I'm wanting to build into the program is it a type of one of the other classes that I have? And in this case, an admin user is definitely a type of user. Our user is our abstract high-level type of class, it's a blueprint that has elements such as email, first name, and last name, and then it has a function built into it. This function is something that should be globally available to all users.

But let's create an admin class, so I'm gonna say class admin user and the way that you declare that one type a class inherits from another is right here in the top line where you say class you give the name of the class and then you pass in using parentheses the element so whatever class you want it to inherit from. So I'm gonna say class admin user inherits from user.

class AdminUser(User):

And now I can create a custom function. So imagine that you want the admin users and admin users only to be able to know how many active users are on the site. I could say def active_users and it is going to just take in self and then for right now, we're just going to hard code a number in.

class AdminUser(User):
    def active_users(self):
        return '500'

so that is how you create a basic class and then how you create a class that inherits from that. So now let's test this out I'm going to create a few instances, one is going to be of an admin user the other one's going to be a regular user.

I'm gonna say Tiffany admin user and she's going to need to have the same elements and same attributes that are inside the User class so she's going to have to have Tiffany@devcamp.com and then she's going to have to have a first name and then a last name just like how you'd expect. And then I'm going to create another user called Kristine who's just going to be a regular user. So this one will be Kristine@devcamp.com and she'll have a first name and a last name.

So now let's actually use these new instances. So if I want to call print and then Tiffany and I'm going to call the instance right here from here I can print out and have access to that active users method. So I'm going to hit save here and let's run this code you can see it prints out five hundred now.

If I try to do the same thing with Kristine so if I have the user Kristine and I try to allow her to access that active users method I'm going to get an error. And it says AttributeError User object has no attribute active user.

So this is it doing exactly what we want. We've created a specialized class that has all of the attributes and all the behavior of the parent class. So Tiffany when we created it, it needed an email, a first name, and a last name. Now also if I come down here and I call the greeting function. So if I say greeting here for Tiffany even though Tiffany is an admin user and there is no greeting function inside of this class. If I run this again you'll see that it prints out the full greeting.

So it says Hi Tiffany Hudgens, and the reason why this is possible is because of inheritance. Whenever we use inheritance the way it works is that the child classes like AdminUser these specialized classes have access to all of the same attributes and behavior as the parent class. And then you can simply add on any specialized attributes or behavior into that child class the way we did right here with the admin user.

So in review, that is the basic introduction to how to work with inheritance in Python.

Code

class User:
  def __init__(self, email, first_name, last_name):
    self.email = email
    self.first_name = first_name
    self.last_name = last_name

  def greeting(self):
    return f'Hi {self.first_name} {self.last_name}'

class AdminUser(User):
  def active_users(self):
    return '500'


tiffany = AdminUser('tiffany@devcamp.com', 'Tiffany', 'Hudgens')

kristine = User('kristine@devcamp.com', 'Kristine', 'Hudgens')

print(tiffany.active_users())
print(tiffany.greeting())
print(kristine.active_users())