Setting Up With __init__

Adding attributes one by one after creating an object gets tedious and easy to forget. The __init__ method fixes that. It runs automatically the moment you create an object, so you can hand in the starting values right away.

The first parameter of every method is always self, which is the object being built. You store data on it with self.attribute = value.

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

rex = Dog("Rex", 3)
print(rex.name)   # Rex
print(rex.age)    # 3

Notice you pass "Rex" and 3 when creating the object, but you do not pass self yourself. Python fills self in for you. These values stored on self are called instance attributes, and each object gets its own copy.

a = Dog("Ada", 2)
b = Dog("Bo", 7)
print(a.name, b.name)   # Ada Bo

You can also give a parameter a default so it is optional:

class Dog:
    def __init__(self, name, age=0):
        self.name = name
        self.age = age