Class methods and cls usage in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class method modifying class variable

What is the output of this code when calling Dog.increment_legs() followed by print(Dog.legs)?

Python

class Dog:
    legs = 4

    @classmethod
    def increment_legs(cls):
        cls.legs += 1

Dog.increment_legs()
print(Dog.legs)

A 5

B 4

C AttributeError

D TypeError

💡 Hint

Remember that class methods receive the class as the first argument and can modify class variables.

The class method increment_legs increases the class variable legs by 1. Initially, legs is 4, so after calling the method, it becomes 5.

❓ Predict Output

intermediate

Output of class method creating instance

What will be printed when running this code?

Python

class Person:
    def __init__(self, name):
        self.name = name

    @classmethod
    def from_full_name(cls, full_name):
        first_name = full_name.split()[0]
        return cls(first_name)

p = Person.from_full_name('Alice Johnson')
print(p.name)

A Johnson

B Alice Johnson

C Alice

D TypeError

💡 Hint

Look at how the class method processes the full name and what it passes to the constructor.

The class method splits the full name and takes the first part as the name. It then creates a new instance with that first name. So, p.name is 'Alice'.

🧠 Conceptual

advanced

Understanding cls vs self in methods

Which statement best describes the difference between cls in class methods and self in instance methods?

A <code>cls</code> refers to the instance, while <code>self</code> refers to the class.

B <code>cls</code> refers to the class itself, while <code>self</code> refers to the instance of the class.

C <code>cls</code> and <code>self</code> both refer to the instance but in different contexts.

D <code>cls</code> and <code>self</code> are interchangeable and mean the same.

💡 Hint

Think about what each method type is designed to work with.

cls is used in class methods to refer to the class itself, allowing access to class variables and other class methods. self is used in instance methods to refer to the specific object instance.

❓ Predict Output

advanced

Output of class method modifying subclass variable

What will be the output of this code?

Python

class Vehicle:
    wheels = 4

    @classmethod
    def set_wheels(cls, count):
        cls.wheels = count

class Bike(Vehicle):
    wheels = 2

Bike.set_wheels(3)
print(Vehicle.wheels, Bike.wheels)

A 3 2

B 3 3

C 4 2

D 4 3

💡 Hint

Remember that class methods affect the class they are called on, not the parent class.

Calling Bike.set_wheels(3) changes the wheels variable only for the Bike class. The Vehicle class remains unchanged with 4 wheels.

🔧 Debug

expert

Identify the error in class method usage

This code is intended to create a new instance with a default name using a class method. What error will it raise when run?

Python

class Cat:
    def __init__(self, name):
        self.name = name

    @classmethod
    def default_cat(cls):
        return cls(name='Whiskers')

c = Cat.default_cat()
print(c.name)

A No error, prints 'Whiskers'

B TypeError: __init__() got an unexpected keyword argument 'name'

C AttributeError: 'Cat' object has no attribute 'name'

D SyntaxError

💡 Hint

Check the constructor parameters and how the class method calls it.

The class method calls the constructor correctly with the keyword argument 'name'. The instance is created with name 'Whiskers', so printing c.name outputs 'Whiskers'.