一文浅析Python中常用的魔法函数使用指南_Python

python中的魔法函数（magic methods），也称为双下划线方法（dunder methods），是python面向对象编程的核心机制之一。它们以__开头和结尾，允许我们自定义类的行为，使其更符合python的惯用风格。本文将全面介绍这些魔法函数，助你写出更pythonic的代码。

什么是魔法函数

魔法函数是python中一类特殊的方法，它们允许你：

自定义类的内置行为
与python内置函数/操作符交互
实现协议（如迭代器、上下文管理器等）

“python的魔法函数是其数据模型的核心，它们是框架和python交互的方式。” - guido van rossum

常用魔法函数分类与功能

基础魔法函数

方法名	功能描述	示例
__init__	构造函数，初始化对象	obj = myclass()
__str__	返回用户友好的字符串表示	print(obj)
__repr__	返回开发者友好的字符串表示	repr(obj)
__len__	返回容器长度	len(obj)

class book:
    def __init__(self, title, author, pages):
        self.title = title
        self.author = author
        self.pages = pages
    
    def __str__(self):
        return f"'{self.title}' by {self.author}"
    
    def __repr__(self):
        return f"book(title='{self.title}', author='{self.author}')"
    
    def __len__(self):
        return self.pages

book = book("python crash course", "eric matthes", 544)
print(book)        # 'python crash course' by eric matthes
print(repr(book))  # book(title='python crash course', author='eric matthes')
print(len(book))   # 544

比较操作魔法函数

这些方法让你的对象可以使用比较操作符：

class box:
    def __init__(self, weight):
        self.weight = weight
    
    def __lt__(self, other):
        return self.weight < other.weight
    
    def __eq__(self, other):
        return self.weight == other.weight

box1 = box(10)
box2 = box(20)
print(box1 < box2)  # true
print(box1 == box2) # false

算术操作魔法函数

方法	对应操作符
__add__	+
__sub__	-
__mul__	*
__truediv__	/
__pow__	**

class vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __add__(self, other):
        return vector(self.x + other.x, self.y + other.y)
    
    def __str__(self):
        return f"vector({self.x}, {self.y})"

v1 = vector(1, 2)
v2 = vector(3, 4)
print(v1 + v2)  # vector(4, 6)

迭代器与容器协议

迭代器魔法函数

class countdown:
    def __init__(self, start):
        self.start = start
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.start <= 0:
            raise stopiteration
        self.start -= 1
        return self.start + 1

for i in countdown(5):
    print(i, end=' ')  # 5 4 3 2 1

容器魔法函数

方法	功能
__getitem__	获取元素 (obj[key])
__setitem__	设置元素 (obj[key] = value)
__delitem__	删除元素 (del obj[key])
__contains__	成员检查 (key in obj)

class sparselist:
    def __init__(self, size):
        self.size = size
        self.data = {}
    
    def __getitem__(self, index):
        if index < 0 or index >= self.size:
            raise indexerror("index out of range")
        return self.data.get(index, 0)
    
    def __setitem__(self, index, value):
        if index < 0 or index >= self.size:
            raise indexerror("index out of range")
        self.data[index] = value
    
    def __contains__(self, value):
        return value in self.data.values()

sparse = sparselist(10)
sparse[3] = 42
print(sparse[3])      # 42
print(42 in sparse)   # true
print(sparse[4])      # 0

高级魔法函数应用

属性访问控制

class protectedattributes:
    def __init__(self):
        self._protected = "this is protected"
    
    def __getattr__(self, name):
        return f"'{name}' attribute not found"
    
    def __setattr__(self, name, value):
        if name.startswith('_'):
            super().__setattr__(name, value)
        else:
            raise attributeerror(f"cannot set attribute '{name}'")

obj = protectedattributes()
print(obj.nonexistent)  # 'nonexistent' attribute not found
# obj.public = "test"   # raises attributeerror

上下文管理器

class databaseconnection:
    def __enter__(self):
        print("connecting to database...")
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("closing connection...")
        if exc_type:
            print(f"error occurred: {exc_val}")
        return true  # suppress exceptions

with databaseconnection() as conn:
    print("executing query...")
    # raise valueerror("invalid sql")  # would be handled by __exit__

可调用对象

class counter:
    def __init__(self):
        self.count = 0
    
    def __call__(self, increment=1):
        self.count += increment
        return self.count

counter = counter()
print(counter())      # 1
print(counter(5))     # 6

性能优化案例

使用__slots__减少内存占用

class regularpoint:
    def __init__(self, x, y):
        self.x = x
        self.y = y

class slottedpoint:
    __slots__ = ['x', 'y']
    def __init__(self, x, y):
        self.x = x
        self.y = y

import sys
regular = regularpoint(1, 2)
slotted = slottedpoint(1, 2)
print(sys.getsizeof(regular))  # 56 bytes (approx)
print(sys.getsizeof(slotted))  # 32 bytes (approx)

性能提示：__slots__可以显著减少大量实例的内存使用，但会限制动态属性添加【1†source】。

实际应用场景

1. 实现自定义数值类型

class fraction:
    def __init__(self, numerator, denominator):
        self.numerator = numerator
        self.denominator = denominator
    
    def __add__(self, other):
        new_num = self.numerator * other.denominator + other.numerator * self.denominator
        new_den = self.denominator * other.denominator
        return fraction(new_num, new_den)
    
    def __str__(self):
        return f"{self.numerator}/{self.denominator}"

f1 = fraction(1, 2)
f2 = fraction(1, 3)
print(f1 + f2)  # 5/6

2. 构建智能代理对象

class lazyproperty:
    def __init__(self, func):
        self.func = func
        self.name = func.__name__
    
    def __get__(self, obj, type=none):
        if obj is none:
            return self
        value = self.func(obj)
        setattr(obj, self.name, value)
        return value

class circle:
    def __init__(self, radius):
        self.radius = radius
    
    @lazyproperty
    def area(self):
        print("computing area...")
        return 3.14 * self.radius ** 2

c = circle(5)
print(c.area)  # first call: computes and caches
print(c.area)  # subsequent call: returns cached value

最佳实践与建议

一致性原则：实现比较方法时，确保__eq__和__hash__一致【2†source】
文档字符串：为魔法函数提供清晰的文档说明
错误处理：在魔法函数中提供有意义的错误信息
性能考虑：对于性能关键代码，考虑使用__slots__或c扩展
协议完整性：实现协议时，确保所有必要方法都已实现

# 好的实践示例
class goodexample:
    """遵循最佳实践的类实现"""
    
    def __init__(self, value):
        self._value = value
    
    def __repr__(self):
        return f"{self.__class__.__name__}({self._value!r})"
    
    def __str__(self):
        return f"value: {self._value}"
    
    def __eq__(self, other):
        if not isinstance(other, goodexample):
            return notimplemented
        return self._value == other._value
    
    def __hash__(self):
        return hash(self._value)