Python Instance vs Class Variables

Instance vs Class Variables

In Python, variables inside a class can be classified into class variables and instance variables.
The difference lies in how they are shared across objects.

Class Variables

Belong to the class itself.
Shared across all instances of the class.
Changing a class variable affects all objects (unless shadowed by an instance variable).

Example:

class MyClass:
    class_variable = 0  # Class variable
 
    def __init__(self):
        MyClass.class_variable += 1
 
    def print_class_variable(self):
        print(MyClass.class_variable)
 
obj1 = MyClass()
obj2 = MyClass()
 
obj1.print_class_variable()  # Output: 2
obj2.print_class_variable()  # Output: 2

Instance Variables

Belong to a specific object (instance).
Each instance has its own copy.
Changing one instance variable does not affect others.

Example:

class MyClass:
    def __init__(self, name):
        self.name = name  # Instance variable
 
    def print_name(self):
        print(self.name)
 
obj1 = MyClass("John")
obj2 = MyClass("Jane")
 
obj1.print_name()  # Output: John
obj2.print_name()  # Output: Jane

Key Differences

Feature	Class Variable	Instance Variable
Scope	Shared by all instances	Unique to each instance
Declaration	Inside the class, but outside methods	Inside `__init__` or other instance methods
Access	Via `ClassName.variable` or `self.variable`	Always via `self.variable`
Modification Effect	Affects all objects (unless overridden)	Affects only that object

When to Use

Use class variables for values common to all objects (e.g., counter, configuration).
Use instance variables for values unique to each object (e.g., name, age, balance).

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