Why do we need init to initialize a python class

Question

I'm pretty new to OOP and I need some help understanding the need for a constructor in a python class.

I understand init is used to initialize class variables like below:

class myClass():
    def __init__ (self):
        self.x = 3
        print("object created")

A = myClass()
print(A.x)
A.x = 6
print(A.x)

Output:

object created
3
6

but, I could also just do,

class myClass():
    x = 3
    print("object created")

A = myClass()
print(A.x)
A.x = 6
print(A.x)

which prints out the same result.

Could you please explain why we need a constructor or give me an example of a case when the above method will not work?

You can read about class members vs instance members in python. This should improve your understandings. — florin
– florin, Commented Nov 15, 2018 at 11:33
You can pass arguments to the class object and assign those values to class members in the constructor. — Nitin Pawar
– Nitin Pawar, Commented Nov 15, 2018 at 11:34

snakecharmerb · Accepted Answer · 2020-10-05 14:22:47Z

10

Citation: But I can also do

class myClass():
    x = 3
    print("object created")

A = myClass()
print(A.x)
A.x = 6
print(A.x)

No you cannot. There is a fundamental difference once you want to create two or more objects of the same class. Maybe this behaviour becomes clearer like this

class MyClass:
    x = 3
    print("Created!")

a = MyClass() # Will output "Created!"
a = MyClass() # Will output nothing since the class already exists!

In principle you need __init__ in order to write that code that needs to get executed for every new object whenever this object gets initialized / created - not just once when the class is read in.

edited Oct 5, 2020 at 14:22

snakecharmerb

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answered Nov 15, 2018 at 11:39

quant

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1 Comment

stedy Over a year ago

Thank you, this helped my understanding!

solstice333 · Accepted Answer · 2018-11-15 11:42:50Z

5

__init__ is used to initialize the state of multiple instances of a class where each instance's state is decoupled from each other, whereas your second example, without __init__ initializes an attribute that is shared among all instances of a class.

answered Nov 15, 2018 at 11:42

solstice333

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1 Comment

stedy Over a year ago

Okay, my understanding became clearer.. Thank you :)

Sourav Bethi · Accepted Answer · 2020-05-16 14:42:14Z

init() is a default method which is called every time an object is created,so here the variables inside the init are called as instance attributes. the init() is called automatically every time an object is created, data you pass to object when it is created it gets assigned to the instance variables with the init() method here we are simply binding the method and the variables, and each object will have separate copy of these instance variables changing the data in one object will not effect it in another object

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