Python class init – Python __init__() function

Filed Under: Python

Python class init

python class init, python __init__ function
Whenever a beginner starts learning the Python programming language, they come across something like __init__ which usually they don’t fully understand. In this lesson, we will try to understand the use of __init__ completely with good examples. Let’s get started.

Understanding python class init function

Let’s see a short code snippet and see what we’re trying to understand:


class Student(object):

    def __init__(self, something):
        print("Init called.")
        self.something = something

    def method(self):
        return self.something 

my_object = Student('Jetty')

What does the __init__ method do? Why is it necessary? Let’s find out.

What does the python init method do?

When a new instance of a python class is created, it is the __init__ method which is called and proves to be a very good place where we can modify the object after it has been created.

This means that when we create a new instance of the class like:


my_object = Student('Jetty')

In above snippet, when we called Student with ‘Jetty’ (which could be actually anything), it gets passed to the __init__ function as the argument, Jetty. Let’s try to run this script now:
python class init

Is __init__ the constructor?

Actually yes. __init__ is an oop construct. __init__ is the constructor for a class. Just like mentioned above, the __init__ method is called as soon as the memory for the object is allocated. Let’s see what we did above in our snippet:


def __init__(self, something):
    self.something = something

Using self is important because if you don’t and implement your method like:


def __init__(self, something):
    _something = something

The something parameter would be stored in variables on the stack and would be discarded as soon as the __init__ method goes out of scope.

How __init__ works with Inheritance?

When we have a class inheriting from a superclass, __init__ method works the same way. Let us try to demonstrate what happens when we try to initialise a child class:


class User(object):
    def __init__(self, something):
        print("User Init called.")
        self.something = something

    def method(self):
        return self.something 

class Student(User):
    def __init__(self, something):
        User.__init__(self, something)
        print("Student Init called.")
        self.something = something

    def method(self):
        return self.something 

my_object = Student('Jetty')

In above code, when we initialised the Student object, this will be the output which is created when we ran the above program:
python init super class inheritance
So, before the child class, the parent’s class init was called. You can control this by modifying the order in which the init is called for a parent or a child class. Read more at python inheritance.

Conclusion

To summarise, python __init__ is what is called as a constructor in other OOPs languages such as C++ and Java. The basic idea behind this is, it a special method which is automatically called when an object of that Class is created.

Comments

  1. Bill Murray says:

    The “self” keyword in the __init__() tells the python interpreter to make the variables part of the new object. So, if you use self.myvar = ‘foo’, then you’re new object will have a variable called myvar which equals ‘foo’. If you used myvar = ‘foo’ without the self keyword, the __init__ function would have a variable named myvar set to ‘foo’ which would be out of scope and lost when __init__ finished.
    Try it out yourself.

  2. Abhishek Madhu says:

    Why do we need the ‘self’ as a parameter in the __init()__ ?

  3. Pawel says:

    Unfortunately I haven’t understood anything from this article. I don’t know, maybe this was translated from other language but either information and samples are useless and do not bring any clarification.

    1. Pankaj says:

      I don’t think there is anything complex here. Python __init__() function is called when we create an instance of the class. That’s all. I hope it’s clear to you now.

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