Instance vs. Class Methods in Python: When Should You Use \'self\' and \'cls\'?

Diving into the Nuances of Class and Instance Methods: Beyond Self vs. Cls

The Python Enhancement Proposal (PEP) 8 suggests the use of "self" as the first argument in instance methods and "cls" as the first argument in class methods. This distinction stems from the different roles that these methods play in working with instances and classes.

Instance Methods: The Self Advantage

Instance methods are invoked on instances of a class. They typically interact with specific attributes and behavior of that particular instance. The first parameter of these methods is self, which represents the instance on which the method is being called.

For example, the following class defines an "introduce" instance method:

<code class="python">class Inst:
    def __init__(self, name):
        self.name = name

    def introduce(self):
        print("Hello, I am %s, and my name is %s" % (self, self.name))</code>

When we create an instance of the Inst class and call its "introduce" method, the instance itself is passed as the self parameter, allowing us to access its attributes (in this case, the "name" attribute).

Class Methods: Embracing Cls

Class methods, on the other hand, operate on the class itself rather than on individual instances. They allow us to modify or inspect the class structure or behavior. The first parameter of these methods is cls, which represents the class on which the method is being called.

The following example illustrates a class method:

<code class="python">class Cls:
    @classmethod
    def introduce(cls):
        print("Hello, I am %s!" % cls)</code>

This method doesn't require an instance since it doesn't interact with specific object attributes. Instead, it operates on the class itself, providing information about its structure.

Class methods are particularly useful when inheriting from a parent class, as they allow the child class to modify or extend the behavior of the parent class. For instance, the following subclass overrides the "introduce" method of the Cls class:

<code class="python">class SubCls(Cls):
    pass

SubCls.introduce()
# outputs: Hello, I am <class 'SubCls'></code>

By using "cls" as the first parameter, the "introduce" method can be called directly on the subclass, allowing it to define its own behavior while still accessing inherited properties from the parent class.

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