🏷️ Python Class Attributes – Shared State Across Instances

🧲 Introduction – Why Use Class Attributes?

In Python OOP, class attributes are attributes that are shared across all instances of a class. Unlike instance attributes (defined with self), class attributes:

• Belong to the class itself, not to individual objects
• Are stored only once in memory
• Can be accessed or modified using either the class or any of its objects

They are useful when you want shared data or constants across all objects.

🎯 In this guide, you’ll learn:

• What class attributes are
• How they differ from instance attributes
• How to access and modify them
• Use cases and best practices
• Real-world examples and gotchas

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✅ What Are Class Attributes?

A class attribute is defined inside the class but outside all methods.

class Car:
    wheels = 4  # class attribute

    def __init__(self, brand):
        self.brand = brand  # instance attribute

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🔁 Accessing Class Attributes

🔹 Using the class name:

print(Car.wheels)  # 4

🔹 Using an instance:

c1 = Car("Tesla")
print(c1.wheels)  # 4

💡 Even though you’re using an object, it refers to the class attribute if no instance attribute of the same name exists.

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🔄 Modifying Class Attributes

✅ Change it for all instances:

Car.wheels = 6
print(c1.wheels)  # 6

⚠️ Overriding with an instance attribute:

c1.wheels = 8
print(c1.wheels)      # 8
print(Car.wheels)     # 6 (unchanged)

💡 When you assign c1.wheels = 8, it creates an instance attribute that shadows the class attribute.

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📋 Class vs Instance Attributes

Feature	Class Attribute	Instance Attribute
Defined where?	Outside methods	Inside __init__()
Belongs to	Class	Object
Memory	Shared	Unique per object
Accessed by	ClassName.attr or obj.attr	self.attr
Use case	Constants, defaults	Unique object data

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🧪 Real-World Example – Counting Instances

class User:
    count = 0  # class attribute

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

u1 = User("Alice")
u2 = User("Bob")
print(User.count)  # 2

💡 The count attribute is shared and updated across all objects.

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🧱 Example – Shared Configuration

class Config:
    debug = True
    version = "1.0.0"

print(Config.debug)  # True
Config.debug = False
print(Config.debug)  # False

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⚠️ Common Mistakes

❌ Mistake	✅ Fix
Modifying class attribute via object	Always modify using class name
Assuming class attribute is unique per object	Use instance attributes for object-specific data
Using mutable class attributes (e.g., lists) without care	Copy them in __init__ for safety

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📘 Best Practices

✅ Do This	❌ Avoid This
Use class attributes for constants or counters	Store per-object data in class attributes
Access them via ClassName.attr for clarity	Rely on obj.attr when unsure
Document with comments	Leave default values unexplained
Be cautious with mutable class attributes	Use copies inside __init__ if needed

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📌 Summary – Recap & Next Steps

Python class attributes provide a powerful way to share common data across all instances of a class. Use them to define default values, constants, or track instance counts.

🔍 Key Takeaways:

• ✅ Class attributes are shared by all instances
• ✅ Defined at the class level, outside methods
• ✅ Modifying via instance creates a new instance variable
• ✅ Useful for defaults, counters, and configuration flags

⚙️ Real-World Relevance:
Used in web configurations, singleton designs, ORMs, data models, and analytics counters.

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❓ FAQ – Python Class Attributes

❓ Are class attributes shared across all objects?

✅ Yes. They are stored once and shared unless overridden in an instance.

❓ Can class attributes be changed?

✅ Yes, using ClassName.attribute = value. This change will reflect in all objects (unless overridden).

❓ Can class attributes be accessed using self?

✅ Yes, but it’s better to use ClassName.attr for clarity when modifying them.

❓ What happens if I modify a class attribute using an instance?

❌ It creates an instance attribute, not modifying the class one. Always update using the class name.

❓ Should I store mutable defaults in class attributes?

⚠️ Be careful. Shared mutable objects like lists/dicts can cause unexpected behavior across instances. Prefer creating a fresh copy in __init__().

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