self

self is the first argument in methods, which name is agreed upon by Python programmers. When an instance executes a method, it automatically passes itself as the first argument. Let's consider the User class we created earlier as an example:


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class User:
    def __init__(self, name, surname, age):
      # self=bob, name="Bob", surname="Smith", age=11
        self.name = name
      # bob.name = "Bob" (name argument)
        self.surname = surname
      # bob.surname = "Smith" (surname argument) 
        self.age = age
      # bob.age = 11 (age argument)
      
bob = User("Bob", "Smith", 11)
print(bob.name, bob.surname, bob.age)

The __init__ magic method takes the arguments self, name, surname, and age. In the given example, when we create the bob instance, these arguments are passed as values.

Let's use the print() function to examine the passed arguments:


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class User:
    def __init__(self, name, surname, age):
        print("===== INIT ARGUMENTS =====")
        print("self:", self)
        print("name:", name)
        print("surname:", surname)
        print("age:", age)

        self.name = name
        self.surname = surname
        self.age = age
        
bob = User("Bob", "Smith", 11)
print("===== GLOBAL =====")
print("bob:", bob)
print("bob.name:", bob.name)
print("bob.surname:", bob.surname)
print("bob.age:", bob.age)

In the given example, it is evident that the global variable bob and the self parameter inside the __init__ method refer to the same instance. This illustrates the convention in Python where methods receive the instance as the first argument, conventionally named self.

By following this convention, methods have access to the instance attributes and can perform operations or access values specific to that particular instance.

Note
In Python, the self parameter serves as the first argument in methods and represents the instance object that calls the method. By convention, the name self is used, although it could be any valid variable name. This parameter provides access to the attributes and methods of the instance, allowing the method to interact with and manipulate its own data.

For better understanding, let's create a regular function and name it init, which will perform the same actions but will be called separately.


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class User:
    pass

def init(instance, arg1, arg2, arg3):
    instance.name = arg1
    instance.surname = arg2
    instance.age = arg3

bob = User()
init(bob, "Bob", "Smith", 11)
print(bob.name, bob.surname, bob.age)

The init function takes the first argument as an instance and then assigns the received arguments to it. Similarly, __init__ works when creating a class, and self is a necessary element for creating different methods, which we will discuss further.

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 6