Python __init__ and self what do they do?

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

A brief illustrative example

In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:

class MyClass(object):
     i = 123
     def __init__(self):
         self.i = 345

a = MyClass()
print a.i
345
print MyClass.i
123

Answer 2

note that self could actually be any valid python identifier. For example, we could just as easily write, from Chris B's example:

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

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

and it would work exactly the same. It is however recommended to use self because other pythoners will recognize it more easily.

Answer 3

Had trouble undestanding this myself. Even after reading the answers here.

To properly understand the __init__ method you need to understand self.

The self Parameter

The arguments accepted by the __init__ method are :

def __init__(self, arg1, arg2):

But we only actually pass it two arguments :

instance = OurClass('arg1', 'arg2')

Where has the extra argument come from ?

When we access attributes of an object we do it by name (or by reference). Here instance is a reference to our new object. We access the printargs method of the instance object using instance.printargs.

In order to access object attributes from within the __init__ method we need a reference to the object.

Whenever a method is called, a reference to the main object is passed as the first argument. By convention you always call this first argument to your methods self.

This means in the __init__ method we can do :

self.arg1 = arg1
self.arg2 = arg2

Here we are setting attributes on the object. You can verify this by doing the following :

instance = OurClass('arg1', 'arg2')
print instance.arg1
arg1

values like this are known as object attributes. Here the __init__ method sets the arg1 and arg2 attributes of the instance.

source: http://www.voidspace.org.uk/python/articles/OOP.shtml#the-init-method

Answer 4

Just a demo for the question.

class MyClass:

    def __init__(self):
        print('__init__ is the constructor for a class')

    def __del__(self):
        print('__del__ is the destructor for a class')

    def __enter__(self):
        print('__enter__ is for context manager')
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print('__exit__ is for context manager')

    def greeting(self):
        print('hello python')


if __name__ == '__main__':
    with MyClass() as mycls:
        mycls.greeting()

---

$ python3 class.objects_instantiation.py
__init__ is the constructor for a class
__enter__ is for context manager
hello python
__exit__ is for context manager
__del__ is the destructor for a class

Answer 5

In this code:

class Cat:
    def __init__(self, name):
        self.name = name
    def info(self):
        print 'I am a cat and I am called', self.name

Here __init__ acts as a constructor for the class and when an object is instantiated, this function is called. self represents the instantiating object.

c = Cat('Kitty')
c.info()

The result of the above statements will be as follows:

I am a cat and I am called Kitty

Answer 6

1. __init__ is basically a function which will "initialize"/"activate" the properties of the class for a specific object, once created and matched to the corresponding class..
2. self represents that object which will inherit those properties.