Understanding Python Class vs Instance Variables: A Clear Guide

Hello, I am a Python enthusiast sharing a fan's question about class and instance variables.

1. Introduction

A fan studying in France asked in English about the difference between Python class variables and instance variables. The question is straightforward and can be understood with basic English or translation tools.

2. Solution

The answer covers three parts: class attributes, instance attributes, and the reference direction of instance attributes. In the example, counter is a class attribute and __first is an instance attribute. When ExampleClass is instantiated, the __init__ method runs, affecting both attributes. Instance attributes belong to each object individually, while class attributes are shared across all instances. Each time the class is instantiated, ExampleClass.counter increments, which is why the three print statements show the same value.

An additional extension adds self.counter += 1 before ExampleClass.counter += 1 in __init__. This creates an instance attribute counter. The first instantiation sets self.counter to 1 (derived from the class attribute 0 + 1). The second instantiation sees the class attribute already at 1, so self.counter becomes 2, and so on.

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3. Conclusion

The article summarizes that class variables are shared among all instances, while instance variables are private to each object. Changing a class variable affects all objects; changing an instance variable affects only that object.