LM Instance variables vs. class variables

In object-oriented programming, data can belong either to
a single object or to the class itself.

Understanding this difference is essential for writing logical and well-structured programs.

Instance variables

Instance variables store data that belongs to one specific object.

They are defined inside the constructor (__init__) using self.

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

Each object has its own copy of these variables.

Example:

  • one animal has the name "Lion"
  • another animal has the name "Penguin"

Class variables

Class variables store data that belongs to the class as a whole,
not to individual objects.

They are defined inside the class, but outside all methods.

class Animal:
    animal_regions = [
        ["Lion", "Africa"],
        ["Elephant", "Africa"],
        ["Polar Bear", "Arctic"]
    ]

This data exists only once and is shared by all objects of the class.

When should we use class variables?

Use a class variable when:

  • the data is the same for all objects
  • the data represents general or shared knowledge
  • the data should not change per object

In our example:

  • the region of a lion does not depend on a specific lion object
  • it is shared world knowledge
  • therefore, it belongs to the class

Accessing class variables

Class variables can be accessed in two ways:

Animal.animal_regions

or

self.animal_regions

For learning and clarity, it is recommended to access class variables using the class name:

Animal.animal_regions

This makes it explicit that the data belongs to the class, not to the instance.

Why not store this data as an instance variable?

If animal_regions were an instance variable:

  • every object would store its own copy
  • memory usage would increase
  • program logic would be unclear

Class variables avoid duplication and improve structure.

Key takeaway

  • Instance variables belong to individual objects (self)
  • Class variables belong to the class itself
  • Class variables are defined outside methods
  • They are shared by all objects
  • Use them for shared or global knowledge

Understanding class variables is a key step toward
clean and logical object-oriented programming.

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