Classes and objects in Python are a crucial part of the language. You can’t properly learn Python without understanding classes and objects. In this chapter, you will learn:
- how in Python everything is an object
- how to create your own classes and objects in Python
- what inheritance is
When you’re just creating small scripts, chances are you don’t need to create your own classes. But once you start creating larger applications, objects and classes allow you to organize your code naturally.
Table of contents
A look under the hood
Before diving into all the details, we’ll start by taking a look under the hood. I do this because I believe it will give you a much better understanding of these concepts. Don’t let the length of this page discourage you. After reading it thoroughly, and trying the examples yourself, you should have a good understanding of classes and objects in Python. If not… leave a message in the forums and tell me where you got stuck. I’d love to help you through this.
OK; let’s dive in! You probably know the built-in
len() function. It simply returns the length of the object you give it. But what is the length of, say, the number five? Let’s ask Python:
>>> len(5) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: object of type 'int' has no len()
I love errors because they illustrate how Python works internally. In this case, Python is telling us that 5 is an object, and it has no
len(). In Python, everything is an object. Stings, booleans, numbers, and even functions are objects. We can inspect an object in the REPL using the built-in function
dir(). When we try
dir on the number five, it reveals a big list of functions that are part of any number object:
>>> dir(5) ['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', ... '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']
I truncated the list a little for the sake of clarity.
The list starts with these weirdly named functions containing underscores, like
__add__. These are called magic methods, or dunder (short for double underscore) methods. If you look closely, you’ll see that there’s no
__len__ dunder method for objects of type
int. That’s how Python’s
len() function knows that a number does not have a length. All
len() does, is call the
__len__() method on the object you offered it. That’s also why Python complained that “objects of type ‘int’ have no len().”
What are Python Methods?
I casually introduced the word methods here. Let me define it more formally:
- When a function is part of an object, we call it a method.
So if a string does have a length, it must have a
len method, right? Let’s find out!
>>> dir("test") ['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'capitalize', 'casefold', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isascii', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']
Yup, there it is. And since this is a method, we can call it too:
>>> "test".__len__() 4
This is equivalent to
len("test") but a lot less elegant, so don’t do this. It’s just to illustrate how this stuff works.
There’s also a list of other, less magical methods that
dir() revealed to us. Feel free to try a few, like
>>> "test".islower() True
This method checks if the entire string is lower-case, which it is, so Python returns the boolean
True. Some of these methods require one or more arguments, like replace:
>>> 'abcd'.replace('a', 'b') 'bbcd'
It replaces all occurrences of ‘a’ with ‘b’.
What are classes and objects in Python?
Now that we’ve used objects and know that everything in Python is an object, it’s time to define what an object is:
- An object is a collection of data (variables) and methods that operate on that data
Objects and object-oriented programming are concepts that became popular in the early 1990s. Early computer languages, like C, did not have the concept of an object. However, it turned out that objects are an easy to understand paradigm for humans - it can be used to model many real-life situations. Most, if not all, new languages have the concept of objects these days. So what you’re about to learn will conceptually apply to other languages too.
Since objects are the building blocks of the Python language, it’s only logical that you can create objects yourself too. If you want to create your own type of object, you first need to define its methods and what data it can hold. This blueprint is called a class.
- A class is the blueprint for one or more objects
All Python objects are based on a class. When we create an object, we call this ‘creating an instance of a class’. Strings, numbers, and even booleans are instances of a class too. Let’s explore with the built-in function
>>> type('a') <class 'str'> >>> type(1) <class 'int'> type(True) <class 'bool'>
Apparently, there are classes called
bool. These are some of Python’s native classes, but we can build our own class too!
Creating classes and objects in Python
No tutorial is complete without a car analogy, so let’s make a Python class that represents a car. It’s a lot to type in, and you have to start over on each mistake. Feel free to try, but if you want to take a shortcut, I understand. Just copy and paste the following into your Python REPL. Make sure to hit enter twice after pasting it:
class Car: speed = 0 started = False def start(self): self.started = True print("Car started, let's ride!") def increase_speed(self, delta): if self.started: self.speed = self.speed + delta print('Vrooooom!') else: print("You need to start the car first") def stop(self): self.speed = 0 print('Halting')
Don’t worry, we’ll go over this step by step, but let’s first create and use an object of type Car:
>>> car = Car() >>> car.increase_speed(10) You need to start the car first >>> car.start() Car started, let's ride! >>> car.increase_speed(40) Vrooooom! >>> _
An object in Python is always an instance of a class. One class can have many instances. We just created an instance of class
Car(), and assigned it to the (lowercase) variable
car. Creating an instance looks like calling a function ; you’ll learn why later on.
Next, we call one of our car object methods: trying to increase its speed while it’s not started yet. Oops! Only after starting the car, we can increase its speed and enjoy the noise it makes.
Now let’s go over our Car class step by step:
- A class in Python is defined using the class statement, followed by the class’s name (Car). We start an indented block of code with the colon.
- We defined two variables, speed and started. This is data that all instances of this class will have.
- Next, we defined three methods that operate on the variables.
In the definitions of these methods, we encounter something peculiar: they all have an argument called self as their first argument.
What Is self in Python?
Honestly, this is one of Python’s less elegant language constructs if you ask me.
Remember when we were calling the methods on our car object, like
car.start()? We didn’t have to pass the
self variable, even though
start() is defined as
start(self) inside the class.
This is what’s happening:
- When we call a method on an object, Python automatically fills in the first variable, which we call self by convention.
- This first variable is a reference to the object itself, hence its name.
- We can use this variable to reference other instance variables and functions of this object, like
So only inside the class definition, we use
self to reference variables that are part of the instance. To modify the
started variable that’s part of our class, we use
self.started and not just
started. By using
self, it’s made abundantly clear that we are operating on a variable that’s part of this instance and not some other variable that is defined outside of the object and happens to have the same name.
Creating Multiple Objects
Since a class is just a blueprint, you can use it to create multiple objects, just like you can build multiple identical-looking cars. They all behave similarly, but they all have their own data that is not shared between objects:
>>> car1 = Car() >>> car2 = Car() >>> id(car1) 139771129539104 >>> id(car2) 139771129539160
We created two car objects here, car1 and car2, and used the built-in method id() to get their ids. Each object in Python has a unique identifier, so we just proved that we created two different objects from the same class. We can use them independently:
>>> car1.start() Car started, let's ride! >>> car1.increase_speed(10) 'Vrooom!' >>> car1.speed 10 >>> car2.speed 0
We just started
car1 and increased its speed, while
car2 is still halted. Inspection of the speeds confirms these are different cars with different states!