Class methods and cls usage in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class method using the correct decorator.

Python

class MyClass:
    [1]
    def greet(cls):
        return "Hello from class!"

Drag options to blanks, or click blank then click option'

A@classmethod

B@abstractmethod

C@property

D@staticmethod

Attempts:

3 left

💡 Hint

Common Mistakes

Using @staticmethod instead of @classmethod

Forgetting the decorator entirely

Using @property which is for attributes

The @classmethod decorator is used to define a method that receives the class itself as the first argument.

2fill in blank

medium

Complete the class method to return the class name using the cls parameter.

Python

class Animal:
    @classmethod
    def get_class_name([1]):
        return [1].__name__

Drag options to blanks, or click blank then click option'

Acls

Bclass

Cself

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' which is for instance methods

Using reserved words like 'class'

Using 'this' which is not Python syntax

Class methods receive the class as the first parameter, usually named 'cls'.

3fill in blank

hard

Fix the error in the class method to correctly create an instance using cls.

Python

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

    @classmethod
    def create(cls, name):
        return [1](name)

Drag options to blanks, or click blank then click option'

Aname

Bself

CPerson

Dcls

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Person' directly which breaks inheritance

Using 'self' which is not defined in class methods

Using 'name' which is a string, not a class

Inside a class method, use 'cls' to create new instances so it works correctly with subclasses.

4fill in blank

hard

Fill both blanks to define a class method that counts how many instances were created.

Python

class Counter:
    count = 0

    def __init__(self):
        Counter.count += 1

    @[1]
    def get_count([2]):
        return [2].count

Drag options to blanks, or click blank then click option'

Aclassmethod

Bcls

Cstaticmethod

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using @staticmethod which does not receive class or instance

Using 'self' in a class method

Forgetting the decorator

The method must be decorated with @classmethod and receive 'cls' to access class variables.

5fill in blank

hard

Fill all three blanks to create a class method that returns a dictionary with uppercase class name as key and count as value.

Python

class Tracker:
    count = 0

    def __init__(self):
        Tracker.count += 1

    @[1]
    def summary([2]):
        return { [3]: [2].count }

Drag options to blanks, or click blank then click option'

Aclassmethod

Bcls

Ccls.__name__.upper()

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' instead of 'cls' in class method

Using @staticmethod instead of @classmethod

Using a string literal instead of cls.__name__.upper()

Use @classmethod and 'cls' parameter; the key is the uppercase class name accessed by cls.__name__.upper().

Practice

1. What does the cls keyword represent inside a class method in Python?

easy

A. A global variable

B. The class itself

C. A local variable

D. An instance of the class

Solution

1. Step 1: Understand the role of cls in class methods

   Inside a class method, cls refers to the class, not an instance.

2. Step 2: Differentiate cls from self

   self refers to an instance, while cls refers to the class itself.

3. Final Answer:

   The class itself -> Option B

4. Quick Check:

   cls = class [OK]

Hint: Remember: cls means class, self means instance [OK]

Common Mistakes:

• Confusing cls with self
• Thinking cls is a local variable
• Assuming cls is an instance

2. Which of the following is the correct way to define a class method in Python?

easy

A. def method(cls):

B. def method(self):

C. @staticmethod\ndef method():

D. @classmethod\ndef method(cls):

Solution

1. Step 1: Identify the decorator for class methods

   Class methods require the @classmethod decorator above the method.

2. Step 2: Check the method parameter

   Class methods take cls as the first parameter, not self.

3. Final Answer:

   @classmethod\ndef method(cls): -> Option D

4. Quick Check:

   Class method = @classmethod + cls parameter [OK]

Hint: Class methods always use @classmethod and cls parameter [OK]

Common Mistakes:

• Forgetting the @classmethod decorator
• Using self instead of cls
• Defining without any decorator

3. What will be the output of the following code?

class Dog:
    species = 'Canine'

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

print(Dog.get_species())

medium

A. 'Canine'

B. None

C. 'Dog'

D. Error

Solution

1. Step 1: Understand class attribute access via cls

   The class method get_species returns cls.species, which is 'Canine'.

2. Step 2: Check the print statement output

   Calling Dog.get_species() returns 'Canine', which is printed.

3. Final Answer:

   'Canine' -> Option A

4. Quick Check:

   cls.species = 'Canine' [OK]

Hint: Class methods access class variables via cls [OK]

Common Mistakes:

• Expecting instance name instead of class attribute
• Confusing output with class name string
• Thinking it returns None

4. Find the error in this code snippet:

class Cat:
    count = 0

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

Cat.increment()

medium

A. Using count without cls prefix inside method

B. Missing @staticmethod decorator

C. Method should have self parameter

D. Class attribute count is not defined

Solution

1. Step 1: Identify variable usage inside class method

   The method tries to increment count without cls., causing an error.

2. Step 2: Correct usage of class attribute inside class method

   It should be cls.count += 1 to modify the class attribute.

3. Final Answer:

   Using count without cls prefix inside method -> Option A

4. Quick Check:

   Use cls.count to access class variable [OK]

Hint: Always prefix class vars with cls inside class methods [OK]

Common Mistakes:

• Forgetting cls. before class variable
• Using self in class method
• Missing decorator

5. How can you use a class method to create an alternative constructor that creates an object from a string?
Example: Person.from_string('John-25') creates Person('John', 25).
Which code snippet correctly implements this?

hard

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

@staticmethod
def from_string(data):
name, age = data.split('-')
return cls(name, int(age))

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

def from_string(self, data):
name, age = self.split('-')
return Person(name, int(age))

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

@classmethod
def from_string(cls, data):
name, age = data.split('-')
return cls(name, int(age))

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

@classmethod
def from_string(self, data):
name, age = data.split('-')
return cls(name, int(age))

Solution

1. Step 1: Recognize the use of class method as alternative constructor

   The method should be decorated with @classmethod and take cls as first parameter.

2. Step 2: Parse string and return new instance

   Split the string, convert age to int, and return cls(name, int(age)) to create a new object.

3. Final Answer:

   @classmethod with cls parameter returning cls instance -> Option C

4. Quick Check:

   Alternative constructor = @classmethod + cls + return cls(...) [OK]

Hint: Use @classmethod and cls to build alternative constructors [OK]

Common Mistakes:

• Using @staticmethod instead of @classmethod
• Missing cls parameter or using self
• Not returning cls instance