Class methods and cls usage in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class method in Python?

A class method is a method that is bound to the class and not the instance. It receives the class itself as the first argument, usually named <code>cls</code>, allowing it to access or modify class state.

beginner

How do you define a class method in Python?

You define a class method by decorating a method with <code>@classmethod</code> and including <code>cls</code> as the first parameter in the method definition.

intermediate

Why use <code>cls</code> instead of <code>self</code> in a class method?

<code>cls</code> refers to the class itself, allowing the method to access or modify class variables and create new instances. <code>self</code> refers to an instance of the class, used in instance methods.

intermediate

Can a class method create an instance of the class? How?

Yes, a class method can create an instance by calling <code>cls()</code> with appropriate arguments. This is useful for alternative constructors.

beginner

Example: What does this code output?<br><pre>class Dog:
    species = 'Canis'

    @classmethod
    def get_species(cls):
        return cls.species

print(Dog.get_species())</pre>

The output is <code>Canis</code>. The class method <code>get_species</code> accesses the class variable <code>species</code> using <code>cls.species</code>.

What is the first argument of a class method in Python?

A self

B cls

C class

D this

Class methods receive the class itself as the first argument, which is conventionally named cls.

Which decorator is used to define a class method?

A @classmethod

B @staticmethod

C @instance

D @property

The @classmethod decorator marks a method as a class method.

What can a class method access even without an instance?

A Instance variables

B Global variables

C Class variables

D Local variables

Class methods can access class variables via cls, even without an instance.

Which of these is a typical use of a class method?

A To create alternative constructors

B To modify instance attributes

C To handle user input

D To define private methods

Class methods are often used as alternative constructors to create instances in different ways.

What will this code print?<br>

class Cat:
    count = 0

    @classmethod
    def increment(cls):
        cls.count += 1

Cat.increment()
print(Cat.count)

A 0

B Error

C None

D 1

The class method increments the class variable count by 1, so the output is 1.

Explain what a class method is and how the cls parameter is used.

Think about how class methods relate to the class itself, not instances.

Describe a situation where using a class method is better than an instance method.

Consider when you want to create or manage objects without needing an existing object.