Class methods and cls usage in Python - Step-by-Step Execution

Concept Flow - Class methods and cls usage

Define class with @classmethod

↓

Call class method on class or instance

↓

cls parameter receives the class

↓

Access or modify class variables via cls

↓

Return or print results

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End

Class methods receive the class itself as the first argument (cls), allowing access to class variables and methods without needing an instance.

Execution Sample

Python

class Dog:
    count = 0

    def __init__(self, name):
        self.name = name
        Dog.count += 1

    @classmethod
    def how_many(cls):
        return cls.count

print(Dog.how_many())
d1 = Dog('Max')
d2 = Dog('Bella')
print(Dog.how_many())

This code counts how many Dog instances are created using a class method that accesses a class variable.

Execution Table

Step	Action	cls value	Class variable count	Output
1	Call Dog.how_many()	Dog	0	0
2	Create d1 = Dog('Max')	Dog	1
3	Create d2 = Dog('Bella')	Dog	2
4	Call Dog.how_many()	Dog	2	2
5	End	-	-	-

💡 No more code to execute, program ends

Variable Tracker

Variable	Start	After 1	After 2	Final
Dog.count	0	1	2	2
d1.name	-	Max	Max	Max
d2.name	-	-	Bella	Bella

Key Moments - 3 Insights

Why does the class method use cls instead of self?

cls refers to the class itself, not an instance. This allows the method to access class variables like count, as shown in execution_table steps 1 and 4.

Can we call the class method on an instance like d1.how_many()?

Yes, but cls will still refer to the class Dog, not the instance. The method accesses class variables, so it works the same as calling Dog.how_many().

Why does Dog.count increase when creating instances?

In __init__, Dog.count is incremented each time a new Dog is created, updating the class variable tracked by the class method how_many().

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of Dog.count after creating d2?

A 1

B 2

C 0

D 3

💡 Hint

Check the 'Class variable count' column at step 3 in the execution_table.

At which step does Dog.how_many() return 0?

A Step 4

B Step 2

C Step 1

D Step 3

💡 Hint

Look at the 'Output' column in the execution_table for when how_many() is called.

If we add a third Dog instance, what will Dog.count be after creation?

A 3

B 2

C 1

D 4

💡 Hint

Refer to variable_tracker for Dog.count increments after each instance creation.

Concept Snapshot

Class methods use @classmethod decorator.
They receive cls as first parameter, the class itself.
Use cls to access or modify class variables.
Called on class or instance, cls always refers to class.
Useful for tracking data shared by all instances.

Full Transcript

This example shows how class methods work in Python. The class Dog has a class variable count that tracks how many dogs are created. The __init__ method increases count each time a new Dog instance is made. The class method how_many uses cls to access the class variable count and returns it. We call Dog.how_many() before and after creating instances to see the count change. cls always refers to the class Dog, not any instance. This lets class methods access shared data easily.