Master Python’s Instance, Class, and Static Methods: A Practical Guide

In Python's object‑oriented programming, instance variables, class variables, instance methods, class methods, and static methods each serve distinct purposes for organizing data and behavior.

Instance Variables

Instance variables belong to each object created from a class; they are initialized in __init__ and stored separately for every instance.

class Dog:
    def __init__(self, name, color, weight):
        self.name = name
        self.color = color
        self.weight = weight

Creating d1 = Dog('大黄', '黄色', 10) and d2 = Dog('旺财', '黑色', 8) gives each dog its own name, color, and weight values.

Class Variables

Class variables are shared across all instances of the class; there is only one copy in memory.

class Dog:
    dogbook = {'黄色': 30, '黑色': 20, '白色': 0}
    def __init__(self, name, color, weight):
        self.name = name
        self.color = color
        self.weight = weight

The dogbook dictionary can store the count of dogs of each color, and any change to it is reflected in every Dog instance.

Instance Methods

Instance methods receive the instance itself as the first argument ( self) and can access or modify instance variables.

class Dog:
    def __init__(self, name, color, weight):
        self.name = name
        self.color = color
        self.weight = weight

    def bark(self):
        print(f'{self.name}叫了起来')

d1 = Dog('大黄', '黄色', 10)
d1.bark()

Class Methods

Class methods are defined with the @classmethod decorator; the first parameter is conventionally named cls and refers to the class itself.

class Dog:
    dogbook = {'黄色': 30, '黑色': 20, '白色': 0}

    @classmethod
    def dog_num(cls):
        num = 0
        for v in cls.dogbook.values():
            num += v
        return num

d1 = Dog('大黄', '黄色', 10)
print(f'共有{Dog.dog_num()}条狗')
print(f'共有{d1.dog_num()}条狗')  # can also be called via an instance

Class methods are useful for operations that concern the whole class, such as aggregating shared data or providing factory functions.

Static Methods

Static methods are defined with @staticmethod and do not receive self or cls. They behave like regular functions placed inside the class namespace.

class Dog:
    @staticmethod
    def total_weight(dogs):
        total = 0
        for dog in dogs:
            total += dog.weight
        return total

print(f'狗共重{Dog.total_weight([d1, d2])}公斤')

Summary

Instance variables store data unique to each object, while class variables provide shared data for the entire class. Instance methods operate on individual objects, class methods manipulate class‑level state, and static methods offer utility functions that are logically related to the class but independent of its state.

Choosing the appropriate method type leads to clearer, more maintainable Python code.