Self in python Class - I can do it with out it...?

Question

Consider this code:

class example(object):

    def __init__ (): # No self
        test()       # No self

    def test(x,y):   # No self
        return x+y 

    def test1(x,y):  # No self
        return x-y

print(example.test(10,5))
print(example.test1(10,5))

15
5

This works as expected. I believe I can write a whole program not using self. What am I missing? What is this self; why is it needed in some practical way?

I have read a lot about it - (stack, Python documentation), but I just don't understand why it's needed, since I can obviously create a program without it.

Answer 1

You can perfectly create a program without it. But then you'd be missing one of the key features of classes. If you can do without self, I'd argue you can do without classes and just do something purely with functions :)

Classes allow you to create objects which have a PROPERTY associated to them, and self allows you to access those values. So say you have a square.

g code:

class Square(object):

    def __init__ (self, length, height):
        self.length = length # THIS square's length, not others
        self.height = height # THIS square's height, not other

    def print_length_and_height(self):
        print(self.length, self.height) # THIS square's length and height



square1 = Square(2,2)
square2 = Square(4,4)
square1.print_length_and_height() # 2 2
square2.print_length_and_height() # 4 4

Now, this example is quite silly, of course, but i think it shows what SELF specifically is for: it refers to the particular instance of an object.

By all means, if you don't see the point to it, just do away with classes and just use functions, there nothing wrong with that.

Answer 2

You haven't utilised a class or object properly. Cutting out the garbage code, your program reduces to:

def test(x,y): #No class
    return x+y 

def test1(x,y): #No class
    return x-y

print(example.test(10,5))
print(example.test1(10,5))

Output:

15
5

Your "class" is no more useful than if you wrapped your program in the nested structures:

if True:
    for i in range(1):
        ...

---

A proper object will have attributes (data fields) and functions that operate on that data (see below). Your code has an empty object; hence, you have nothing on which to operate, no need for self, and no need for a class at all.

Rather, use a class when you need to encapsulate a data representation and associated operations. Below, I've reused some of your code to make example do some trivial complex number work. There are many extensions and improvements to make in this; I kept it relatively close to your original work.

class example(object):

    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __repr__(self):
        sign = ' + ' if self.b >= 0 else ' - '
        return str(self.a) + sign + str(abs(self.b)) + 'i'

    def add(self, x): 
        self.a += x.a
        self.b += x.b

    def sub(self, x): 
        self.a -= x.a
        self.b -= x.b

complex1 = example(10, 5)
complex2 = example(-3, 2)
complex1.add(complex2)
print(complex1)
complex2.sub(complex1)
print(complex2)

Output:

7 + 7i
-10 - 5i

Answer 3

Are you familiar with Object-Oriented Paradigm?
If you don't you should check it. Python is a Object-Oriented Language and self lets you define your object properties.
An example:
You have a class named Vehicle. A vehicle could be a bike, a car, even a plane. So something you can include is a name and a type.

class Vehicle():
  def init(self, name, type): # Constructor
    self.name = name
    self.type = type
  def info(self):
    print("I'm a ")
    print(self.name)

That's all, now you have a vehicle with name and type. Every instance of Vehicle would have a name and a type different or not and every intance can access its own variables. I'm sorry I can't explain it better. Firstable you need to know Object-Oriented Paradigm knowledge. Please comment my answer if you have doubts & I'll answer you or give a link where it comes explained better.