oop - What __init__ and self do in Python?

Question

I'm learning the Python programming language and I've came across something I don't fully understand.

In a method like:

def method(self, blah):
    def __init__(?):
        ....
    ....

What does self do? What is it meant to be? Is it mandatory?

What does the __init__ method do? Why is it necessary? (etc.)

I think they might be OOP constructs, but I don't know very much.

Answer 1

In this code:

class A(object):
    def __init__(self):
        self.x = 'Hello'

    def method_a(self, foo):
        print self.x + ' ' + foo

... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...

a = A()               # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument

The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.