init self python example

What do __init__ and self do in Python? [duplicate]

stackoverflow.com › questions › 625083 › what-do-init-and-self-do-in-python

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.

Answer from Chris B. on Stack Overflow

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oop - What do __init__ and self do in Python? - Stack Overflow

Top answer

1 of 16

719

In this code:

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

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

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

2 of 16

318

Yep, you are right, these are oop constructs.

__init__ is the constructor for a class. The self parameter refers to the instance of the object (like this in C++).

class Point:
    def __init__(self, x, y):
        self._x = x
        self._y = y

The __init__ method gets called after memory for the object is allocated:

x = Point(1,2)

It is important to use the self parameter inside an object's method if you want to persist the value with the object. If, for instance, you implement the __init__ method like this:

class Point:
    def __init__(self, x, y):
        _x = x
        _y = y

Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope. Setting those variables as self._x and self._y sets those variables as members of the Point object (accessible for the lifetime of the object).

N.B. Some clarification of the use of the word "constructor" in this answer. Technically the responsibilities of a "constructor" are split over two methods in Python. Those methods are __new__ (responsible for allocating memory) and __init__ (as discussed here, responsible for initialising the newly created instance).

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MicroPyramid

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self and __init__ in Python: A Complete Guide | MicroPyramid

June 9, 2026 - In Python, self is the instance of the class itself — the specific object you are working with — and Python passes it automatically as the first argument to every instance method. __init__ is the method Python runs right after a new object is created so you can set up its starting attributes; it is an initializer, not a constructor.

Discussions

Can Someone explain to me what is self and init in python?

Sorry to comment on such an old post, but I thought I'd respond anyway just in case anyone else stumbles across this. I had a very hard time understanding self at first too. This video is what finally made it 'click' for me, and hundreds of people in the comments said the same, so I highly suggest you give it a watch. You can skip to the part where he's using self if you'd like, but I think watching the whole video helps provide more context. That whole series is an excellent watch in general if you want to learn OOP. Why use 'self'? Imagine you had the following empty class: class Employee: pass You could assign a first_name and last_name variable to a new instance of this class like so: emp_1 = Employee() emp_1.first_name = "John" emp_1.last_name = "Doe" This is perfectly valid code, but imagine if you wanted to create many more employees with more variables such as email, phone_number, etc., for each. It would take up a lot of lines and there would also be repeated code. A better approach would instead be to define an __init__ method like so: class Employee: def __init__(self, first, last): self.first_name = first self.last_name = last I'll explain the above in a moment, but just know that we can now create new employees like this: emp_2 = Employee("Johnny", "Doey") emp_3 = Employee("Mark", "Smith") emp_4 = Employee("Jack", "Walker") Instead of like this: emp_2 = Employee() emp_2.first_name = "Johnny" emp_2.last_name = "Doey" emp_3 = Employee() emp_3.first_name = "Mark" emp_3.last_name = "Smith" emp_4 = Employee() emp_4.first_name = "Jack" emp_4.last_name = "Walker" The first way is much nicer, right? Of course, this is only 1 of the many benefits of using self, but I think it's a good example to start with nevertheless. So what actually is init and 'self'? __init__ (simply meaning 'initialize') is a special method that is called immediately after an instance of a class is created. It's the equivalent of the constructor in other languages such as C#. The self parameter just refers to a specific instance of the object itself. It does not need to be called 'self', but doing so is a very popular convention. You could technically replace self with something like emp in this case, if you wanted. You might be wondering "Where is this self parameter even coming from? I don't see it being passed in as an argument anywhere?". Well, when you instantiate a new object in Python, self is automatically passed through as the first argument, so you do not need to include it yourself. However, since self is kind of 'secretly' being passed in, this means that it now needs to be accepted as the first parameter in the __init__ method. Again, self refers to whichever object you are currently instantiating. In the below example, it refers to emp_1 , since that's the name of our variable. emp_1 = Employee("John", "Doe") In the above example, when emp_1 is instantiated, the __init__ method assigns "John" to first_name and "Doe" to last_name for the emp_1 object (self in this case), since those are the arguments that were passed through during instantiation. def __init__(self, first, last): self.first_name = first self.last_name = last More on reddit.com

r/learnpython

261

November 29, 2022

Python class __init__ and self

My question what do self an init mean in python class? like this: class Complex: def __init__(self, realpart, imagpart): self.r = realpart self.i = imagpart so why I have to do that? And what do that mean? More on discuss.python.org

discuss.python.org

October 4, 2024

Understanding classes, init and self

Could you explain this code excerpt? I am learning and trying to get in touch with the logic of classes · The __init__ method initialises a new instance (object) of the class NewWindow. The actual creation of the instance is done by a separate method __new__, before __init__ is called. More on discuss.python.org

discuss.python.org

February 16, 2024

What does __init__ and self do in python?

What does __init__ and self do in Python? __init__ is used to initialise the created class instance, in practice this mostly means setting the attributes the methods expect it to have. self refers to the current instance, although the name itself isn't special in any way whatsoever - you could use whatever valid identifier you wanted, although I don't particularly recommend deviating from established practices here because we're used to self. As for why your code doesn't do what you think, you never called that method. Random_Player = Game('Male', 19) Random_Player.score() As a side note I recommend reading PEP-8, PEP-257, and PEP-484 as the official style guide. More on reddit.com

r/learnpython

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January 19, 2024

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Hello. If you define a function inside a class, it’s called a method. Methods are used by instances of a class. class Demo: def method(self): print(‘test’) demo = Demo() demo.method() # this line above is shorthand for this: Demo.method(self=demo) It’s basically a ‘template’ and shorthand to make useful functions for class instance objects. Init is run each time an object is initialized. You always pass self as the first argument and you’d typically assign any other keyword arguments to the self. class Demo: def __init__(self, name, age): self.name = name self.age = age def __repr__(self): return f’This object is named {self.name} and is of age {self.age}’ demo_person = Demo(name=‘Steve’, age= 31) print(demo_person) So the general idea is, provide all the information your object needs in init. Save references to the object, self is just the naming convention. You can change it (please don’t actually use this fact anywhere). And in my example, I used another special method, repr and made use of the name and age passed into init, by using self.name, self.age. Don’t overthink the syntax. It’s just some boilerplate. Focus on the ideas.

Medium

medium.com › @siddharthchobhe › understanding-init-and-self-in-python-with-simple-examples-72fd08b19dc3

🔍 Understanding __init__ and self in Python (with Simple Examples) | by Siddharth Chobhe | Medium

April 9, 2025 - Its main job is to initialize the object’s attributes — that is, to set them up with the data they need. class MyClass: def __init__(self, arg1, arg2): self.arg1 = arg1 self.arg2 = arg2

Great Learning

mygreatlearning.com › blog › it/software development › python __init__: an overview

Python __init__: An Overview

October 14, 2024 - Here's an example to illustrate the usage of __init__: class MyClass: def __init__(self, name, age): self.name = name self.age = age def display_info(self): print(f"Name: {self.name}") print(f"Age: {self.age}") # Creating an instance of MyClass ...

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medium.com › @ppraveen4a8 › self-and-init-method-in-python-3e55af7d362b

Self and InIt Method in Python. In this blog will discuss about why do… | by Praveen Kumar | Medium

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c-sharpcorner.com › blogs › usage-of-init-and-self-in-python-classes

Usage of __init__ and Self in Python Classes

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W3Schools

w3schools.com › PYTHON › python_class_init.asp

Python __init__() Method

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index.dev › blog › what-is-init-in-python

Master Python Objects: The Ultimate Guide to the __init__ Method

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pythontutorial.net › home › python oop › python __init__

Python __init__

March 31, 2025 - Since Python will automatically ... defines the Person class with the __init__() method: class Person: def __init__(self, name, age): self.name = name self.age = age person = Person('John', 25) print(f"I'm {person.name}. I'm {person.age} years old.") Code language: Python ...

reddit.com › r/learnpython › what does __init__ and self do in python?

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January 19, 2024 - You can also store your books into objects in Python, and each object would have some attributes that are important to know. First, you make the blueprint of the object (the class) and then you start creating new books: ... class Book: def __init__(self, title, author, genre): self.title = ...

Mimo

mimo.org › glossary › python › init-function

Python __init__() Function: Syntax, Usage, and Examples

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Init in Python | Python Init Class | Python _Init_() Function

October 8, 2021 - Get Placement Oriented Python Training from Industry Experts with our Python Training in Chennai · >>> class cylinder def init_(self, height, radius) self.height = height self.radius = radius def get volume of_cylinder(self) return (self.height ...

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__init__ and self Method in Python Class

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