Python class example: __init_

A class is a template for making objects. Each object made from the class (an instance) has its own data, called attributes, and shares the functions defined in the class, called methods.

The program

python

class Dog:
    def __init__(self, name, breed):
        self.name = name
        self.breed = breed
        self.tricks = []

    def bark(self):
        return f"{self.name} says: Woof!"

    def learn(self, trick):
        self.tricks.append(trick)


rex = Dog("Rex", "Labrador")
bella = Dog("Bella", "Beagle")

rex.learn("sit")
rex.learn("roll over")

print(rex.bark())
print(f"{rex.name} the {rex.breed} knows: {rex.tricks}")
print(f"{bella.name} the {bella.breed} knows: {bella.tricks}")

output

Rex says: Woof!
Rex the Labrador knows: ['sit', 'roll over']
Bella the Beagle knows: []

How it works

Dog("Rex", "Labrador") creates a new object and then calls __init__ with it. __init__ sets up the starting attributes.
self is the object the method is working on. When you write rex.learn("sit"), Python calls Dog.learn(rex, "sit"), so inside the method self is rex.
self.tricks = [] is inside __init__, so every dog gets its own new list. That is why teaching Rex a trick does not change Bella.

A readable print

Printing the object itself gives a default text such as <__main__.Dog object at 0x7f...>. Add a __repr__ method to choose what is shown:

python

class Dog:
    def __init__(self, name, breed):
        self.name = name
        self.breed = breed

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


print(Dog("Rex", "Labrador"))

output

Dog('Rex', 'Labrador')

Create a class in Python

The program

How it works

A readable print