Python Classes and Objects: init and Instances

A class is a blueprint for objects, the heart of object-oriented programming (OOP) in Python. Each object made from it is an instance with its own data.

A method called __init__ sets up each new instance. Read Functions and Dictionaries first.

Define a class and create an object

class Player:
    def __init__(self, name, score):
        self.name = name
        self.score = score

ada = Player("Ada", 12)
print(ada.name)
print(ada.score)

Player("Ada", 12) creates a new object and runs __init__ on it. Inside __init__, self is that new object. self.name = name stores a value on the object. A value stored this way is an attribute. Read it back with a dot: ada.name.

Make several instances

Every instance keeps its own attributes. Changing one object leaves the others alone.

class Player:
    def __init__(self, name, score):
        self.name = name
        self.score = score

ada = Player("Ada", 12)
ben = Player("Ben", 7)
ben.score = 9
print(ada.name, ada.score)
print(ben.name, ben.score)

Ada still has 12. Only Ben's score changed to 9.

Forgetting self in `init`

Without self., the value goes into a local variable. It disappears when __init__ ends, so the object never gets the attribute.

class Player:
    def __init__(self, name):
        player_name = name  # a local variable, not an attribute

ada = Player("Ada")
print(ada.name)  # this line raises AttributeError

The fix is self.name = name.

Print an object nicely with `repr`

Printing a plain object shows something like <__main__.Player object at 0x…>. Add a __repr__ method that returns the text you want instead.

class Player:
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def __repr__(self):
        return f"Player({self.name!r}, {self.score})"

print(Player("Ada", 12))
print([Player("Ada", 12), Player("Ben", 7)])

Lists of objects print with it too. __repr__ is a method: a function that belongs to the class. Methods and self covers them.

Note: Name classes in CapWords, like Player or BankAccount. Name attributes in snake_case, like high_score.

See also: Methods and self, Inheritance.

Your turn

Finish __init__ so each Book stores its title and pages.

class Book:
    def __init__(self, title, pages):
        # add two lines: store title and pages on self
        pass

book = Book("Dune", 412)
print(book.title)
print(book.pages)

Expected output

Dune
412

Practise this in the free Python course, where every exercise checks your code instantly.

Python Classes and Objects: __init__ and Instances