Classes and objects

So far you've used objects without making your own: strings, lists and dictionaries are all objects, with data and methods bundled together. A class is a blueprint for making your own kind of object.

Class and object

• A class is the blueprint: "a dog has a name and can bark".
• An object (or instance) is one thing built from it: Rex, Bella.

class Dog:
    def bark(self):
        print("Woof!")

rex = Dog()        # create an object (an instance of Dog)
bella = Dog()
rex.bark()
bella.bark()
print(type(rex))

Class names use PascalCase: Dog, BankAccount, ShoppingCart.

Objects have their own data

Each object can store its own attributes:

class Dog:
    pass               # an empty class for now

rex = Dog()
rex.name = "Rex"
rex.age = 3

bella = Dog()
bella.name = "Bella"
bella.age = 5

print(rex.name, rex.age)
print(bella.name, bella.age)

Setting attributes from outside like this works, but it's fragile: nothing guarantees every dog has a name. The next lesson's __init__ method fixes that.

Why classes?

Without classes, data about one thing ends up scattered across separate variables and functions that have to be kept in step:

# without a class: three parallel pieces of data
names = ["Asha", "Leo"]
balances = [500, 1200]

def deposit(index, amount):
    balances[index] += amount

deposit(0, 100)
print(names[0], balances[0])

With a class, the data and the behaviour that belongs to it live together:

class Account:
    def __init__(self, owner, balance):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount

asha = Account("Asha", 500)
asha.deposit(100)
print(asha.owner, asha.balance)

Don't worry about __init__ and self yet; they're explained next. The point is the shape: each Account carries its own owner and balance, and knows how to deposit into itself.

Everything is an object

print(type(42), type("hi"), type([1, 2]))
print("hello".upper())         # a method of the str class
print((3.5).is_integer())      # even numbers have methods