Class attributes in Python - Step-by-Step Execution

Concept Flow - Class attributes

Define Class with Class Attribute

↓

Create Instance 1

→Access Class Attribute

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Create Instance 2

→Access Class Attribute

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Modify Class Attribute via Class

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All Instances See Updated Value

Class attributes are shared by all instances of a class. Changing them via the class affects all instances.

Execution Sample

Python

class Dog:
    species = "Canine"

dog1 = Dog()
dog2 = Dog()
print(dog1.species)
print(dog2.species)

This code defines a class attribute 'species' and shows that two instances share it.

Execution Table

Step	Action	Variable/Attribute	Value	Note
1	Define class Dog	Dog.species	"Canine"	Class attribute set
2	Create dog1 instance	dog1.species	"Canine"	Instance accesses class attribute
3	Create dog2 instance	dog2.species	"Canine"	Instance accesses class attribute
4	Print dog1.species	Output	"Canine"	Prints class attribute
5	Print dog2.species	Output	"Canine"	Prints class attribute
6	Modify Dog.species = "Wolf"	Dog.species	"Wolf"	Class attribute changed
7	Print dog1.species	Output	"Wolf"	Instance sees updated class attribute
8	Print dog2.species	Output	"Wolf"	Instance sees updated class attribute

💡 All instances share the class attribute; changes via class reflect on all instances.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
Dog.species	"Canine"	"Canine"	"Canine"	"Wolf"	"Wolf"
dog1.species	N/A	"Canine"	"Canine"	"Wolf"	"Wolf"
dog2.species	N/A	N/A	"Canine"	"Wolf"	"Wolf"

Key Moments - 3 Insights

Why do dog1 and dog2 both show the same species value?

Because species is a class attribute shared by all instances, as shown in steps 2 and 3 of the execution_table.

What happens if we change the class attribute via the class itself?

All instances see the updated value, as shown in step 6 where Dog.species changes, and steps 7 and 8 where dog1 and dog2 reflect the new value.

If we change the attribute on one instance, does it affect others?

No, changing an attribute on an instance creates an instance attribute that hides the class attribute for that instance only. This example does not show that, but the execution_table clarifies class attribute sharing.

Visual Quiz - 3 Questions

Look at the execution_table, what is dog1.species after step 3?

A Undefined

B "Wolf"

C "Canine"

D Error

💡 Hint

Check the value of dog1.species in the variable_tracker after step 3.

At which step does the class attribute Dog.species change?

A Step 6

B Step 2

C Step 4

D Step 8

💡 Hint

Look at the Action column in the execution_table for the modification of Dog.species.

If we set dog1.species = "Fox", what would dog2.species be?

A "Fox"

B "Wolf"

C "Canine"

D Error

💡 Hint

Remember class attributes are shared unless overridden on an instance; see key_moments for explanation.

Concept Snapshot

Class attributes are variables shared by all instances.
Defined inside the class but outside methods.
Instances access them unless overridden.
Changing via class changes for all instances.
Changing on instance creates instance attribute.

Full Transcript

This visual trace shows how class attributes work in Python. We define a class Dog with a class attribute species set to "Canine". When we create two instances dog1 and dog2, both access the same class attribute. Printing species from either instance shows "Canine". When we change Dog.species to "Wolf", both instances see the updated value. This is because class attributes are shared by all instances. Changing the attribute on the class affects all instances, but changing it on one instance only affects that instance. This helps understand how data can be shared or customized in classes.