this keyword in JavaScript and self keyword in Python: Any Difference?

We use this in JavaScript and self in Python. Are they same? This question was haunting me for some time. Finally after some digging I came up with a solution. Hope you will love it.

Personally I love examples. So without further ado, let's jump into some examples.

Here is some JS code with this:

function Love(partnerName, myName) {
    this.partnerName = partnerName;
    this.myName = myName;
    this.get_married = function () {
      return `${this.partnerName} and ${this.myName} got married.`
    }
}

const couple1 = new Love('Ross', 'Rachel')
const couple2 = new Love('Chandler', 'Monica')

console.log(couple1.get_married())
console.log(couple2.get_married())

Here is the same code translated to Python:

class Love:
    def __init__(self, partnerName, myName):
        self.partnerName = partnerName
        self.myName = myName

    def get_married(self):
        return f"{self.partnerName} and {self.myName} got married."

couple1 = Love("Ross", "Rachel")
couple2 = Love("Chandler", "Monica")

print(couple1.get_married())
print(couple2.get_married())

Now, what do you think their output would be? Would there be any difference? Think about it for at least one minute.

So the output for both the JS code and Python code would be:

Ross and Rachel got married.
Chandler and Monica got married.

Now we see that both this and self acts similarly. Yes! There is no difference.

But...

I lied, partially. Though their output is the same, this and self are not quite same.

Value of this:

The value of this depends on the context in which it appears. This context can be a function , class or global(Window for browser, global for Node.js).

For easier understanding, the value of this depends on how the function is called.

Note: JavaScript functions can be called in four ways! Such as:

• Calling a function as a function.
• Calling a function as method.
• Calling a function as constructor.
• Calling a function by call and apply

Value of self:

On the other hand, in Python, the value of self depends on the instance created from the class. In this case, the object created from the couple1 instance, is the value of self. And couple2 is the value of self for the second object.

We can prove it by the following code:

class Love:
    def __init__(self, partnerName, myName):
        self.partnerName = partnerName
        self.myName = myName

    def get_married(self):
        print("id of self: ",id(self)) # Notice this line. It prints the id of "self".
        return f"{self.partnerName} and {self.myName} got married."

couple1 = Love("Ross", "Rachel")
couple2 = Love("Chandler", "Monica")

print("id of couple1: ", id(couple1)) # Notice this line. It prints the id of "couple1".
print(couple1.get_married())

print("id of couple2: ", id(couple2)) # Notice this line. It prints the id of "couple2".
print(couple2.get_married())

The output would be:

id of couple1:  140337639399376
id of self:  140337639399376 # This number is the same as the above number.
Ross and Rachel got married.
id of couple1:  140337640683984
id of self:  140337640683984 # This number is the same as the above number.
Chandler and Monica got married.

If you look closely to the ids, you will notice its the same for self and the object instances. This proves that self is actually denoting the instances(couple1 and couple2)

Differences:

1. First of all, this is a reserved keyword. self is not. You can use anything in lieu of self. But it has to be the first parameter.
2. this is dynamically bound(runtime bound), meaning it changes its behavior based on how the function is called. There are many ways to call a function in JavaScript. On the other hand, self in Python is statically bound. It means the value of self is determined by the instance of the class from where the method is called from.

These are the fundamental differences between them. If you have any further query, feel free to comment.

Happy Coding!

-- This article was originally published in DEV Community(dev.to).