Python self Keyword Tutorial

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In this section, we will learn about the keyword self in Python.

Note: We are assuming you already familiar with the Python Class and Object in general.

What is self in Python

The `self` keyword is used as the first parameter in class methods and it refers to the object that invokes those methods.

Basically, by default and when just creating a method, the `self` keyword does not refer to anything! But when we create an object from a class and then use that object to call a method, then the object becomes the reference of the `self` method in the target method that is being called.

Now, using the `self` keyword, we can access and call other methods and attributes of the object that is calling a method.

For example, if the target object has an attribute named `firstName` or `lastName` for example, then we can access those attributes using `self.firstName` or `self.lastName` as the keyword `self` refers the target object.

Note that by default, there’s no need to use the `self` keyword when we want to call an attribute or method of a class! We can simply call these values without the need to use the `self` keyword.

But one of the times that this `self` keyword becomes handy is when we use the same name for different purposes! For example, an object has an attribute named `firstName` and also there’s a method that has a local variable with this name! Now, if we want to call the attribute `firstName` in that method, we need to prefix the target attribute with `self` keyword in order to avoid name collision.

Example: Python class self

class Parent: 

    def __init__ (self, firstName, lastName):
        self.firstName = firstName
        self.lastName = lastName
        
    def setFirstName(self, firstName):
        self.firstName = firstName

How Does Python self Keyword Work?

Note that the `setFirstName` method has a parameter named `firstName`. Now, within the body of this method, the purpose is to assign the argument of this method to the `firstName` attribute of the object. So here if we don’t use the `self` keyword, the execution engine has no idea which `firstName` points to the object’s attribute and which one is the local parameter of the method!

Using the self keyword as the return value of a Method in Python:

Other than using the `self` keyword as the parameter of a method and using it in the body of methods, this keyword can be used as the returned value of a method as well!

In that case, we’re basically returning a pointer to the object that invoked the method from the target method. (This means the return value is not a copy object! But just a pointer to the same object)

Example: python self keyword

class Parent: 

    def __init__ (self, firstName, lastName):
        self.firstName = firstName
        self.lastName = lastName
        
    def returnSelf(self):
        return self  


parent1 = Parent("John","Doe")
parent2 = parent1.returnSelf()
parent3 = parent1

parent2.firstName = "Jack"

print(parent1.firstName)

parent3.firstName = "Omid"

print(parent1.firstName)

Output:

Jack

Omid

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