一、开篇：让+、==对你的对象也有效
当你写3 + 5时，Python实际调用的是(3).__add__(5)。当你写"hello" == "world"时，调用的是"hello".__eq__("world")。Python的每一个运算符背后都有一个魔法方法——你可以为自己的类实现这些方法，让+、-、==、<这些运算符对你的对象生效。
先看最直观的例子：
class Vector:
"""二维向量——支持加减"""
def __init__(self, x, y):
self.x = x
self.y = y
def __add__(self, other):
"""v1 + v2"""
return Vector(self.x + other.x, self.y + other.y)
def __sub__(self, other):
"""v1 - v2"""
return Vector(self.x - other.x, self.y - other.y)
def __repr__(self):
return f"Vector({self.x}, {self.y})"
v1 = Vector(3, 4)
v2 = Vector(1, 2)
print(v1 + v2) # Vector(4, 6)
print(v1 - v2) # Vector(2, 2)
二、算术运算符
2.1 双目运算符
class Money:
"""金额类——支持算术运算"""
def __init__(self, amount, currency="CNY"):
self.amount = amount
self.currency = currency
def __add__(self, other):
if self.currency != other.currency:
raise ValueError("不同货币不能直接相加")
return Money(self.amount + other.amount, self.currency)
def __sub__(self, other):
if self.currency != other.currency:
raise ValueError("不同货币不能直接相减")
return Money(self.amount - other.amount, self.currency)
def __mul__(self, factor):
"""金额 × 数量"""
if isinstance(factor, (int, float)):
return Money(self.amount * factor, self.currency)
return NotImplemented # 不支持的操作返回NotImplemented
def __truediv__(self, divisor):
"""金额 ÷ 数量"""
if divisor == 0:
raise ZeroDivisionError("除数不能为0")
return Money(self.amount / divisor, self.currency)
def __floordiv__(self, divisor):
"""金额 // 数量"""
return Money(self.amount // divisor, self.currency)
def __repr__(self):
return f"Money({self.amount:.2f}, {self.currency})"
# 使用
price = Money(100)
quantity = 3
total = price * quantity
print(total) # Money(300.00, CNY)
print(total / 2) # Money(150.00, CNY)
print(Money(500) - Money(200)) # Money(300.00, CNY)
2.2 反向运算符和增强赋值
class Vector:
def __init__(self, x, y):
self.x, self.y = x, y
def __add__(self, other):
return Vector(self.x + other.x, self.y + other.y)
def __radd__(self, other):
"""反向加法：当左操作数不支持加法时调用"""
# 例如：tuple + Vector
if isinstance(other, (tuple, list)) and len(other) == 2:
return Vector(self.x + other[0], self.y + other[1])
return NotImplemented
def __iadd__(self, other):
"""增强赋值 += """
self.x += other.x
self.y += other.y
return self # 必须返回self！
def __repr__(self):
return f"Vector({self.x}, {self.y})"
v = Vector(3, 4)
# __radd__的例子
# result = (1, 2) + v # 元组不支持 + Vector，Python调用v.__radd__((1,2))
# print(result) # Vector(4, 6)
# __iadd__的例子
v += Vector(1, 1)
print(v) # Vector(4, 5) —— v自身被修改了
三、比较运算符
3.1 等于和不等于
class Person:
def __init__(self, name, id_number):
self.name = name
self.id_number = id_number
def __eq__(self, other):
"""== —— 通过身份证号判断是否同一个人"""
if not isinstance(other, Person):
return NotImplemented
return self.id_number == other.id_number
def __ne__(self, other):
"""!= —— 通常直接反转__eq__即可"""
result = self.__eq__(other)
if result is NotImplemented:
return result
return not result
def __hash__(self):
"""定义了__eq__就必须定义__hash__（用于set和dict的键）"""
return hash(self.id_number)
def __repr__(self):
return f"Person({self.name}, {self.id_number})"
p1 = Person("张三", "110101199001011234")
p2 = Person("张三别名", "110101199001011234") # 同一身份证号
p3 = Person("李四", "110101199501011234")
print(p1 == p2) # True —— 同一个人！
print(p1 == p3) # False
print(p1 != p3) # True
# 可以放入集合——根据__hash__和__eq__去重
people = {p1, p2, p3}
print(len(people)) # 2 —— p1和p2被视为同一个
3.2 大小比较
from functools import total_ordering
@total_ordering # 只需定义__eq__和__lt__，其余自动生成
class Version:
"""语义化版本号"""
def __init__(self, major, minor, patch):
self.major = major
self.minor = minor
self.patch = patch
def __eq__(self, other):
return (self.major, self.minor, self.patch) == \
(other.major, other.minor, other.patch)
def __lt__(self, other):
return (self.major, self.minor, self.patch) < \
(other.major, other.minor, other.patch)
def __repr__(self):
return f"v{self.major}.{self.minor}.{self.patch}"
versions = [Version(2, 0, 1), Version(1, 5, 3),
Version(2, 1, 0), Version(1, 0, 0)]
print(sorted(versions))
# [v1.0.0, v1.5.3, v2.0.1, v2.1.0]
# @total_ordering自动生成了：__le__, __gt__, __ge__
四、容器类运算符
4.1getitem、setitem、len
class Matrix:
"""简单的二维矩阵——支持[][]索引"""
def __init__(self, rows, cols, default=0):
self.rows = rows
self.cols = cols
self._data = [[default] * cols for _ in range(rows)]
def __getitem__(self, index):
"""matrix[row][col]"""
return self._data[index]
def __setitem__(self, index, value):
"""matrix[row] = [val1, val2, ...]"""
if len(value) != self.cols:
raise ValueError(f"每行必须有{self.cols}个元素")
self._data[index] = list(value)
def __len__(self):
"""len(matrix) → 行数"""
return self.rows
def __contains__(self, value):
"""value in matrix —— 检查矩阵中是否有某个值"""
return any(value in row for row in self._data)
def __repr__(self):
return "\n".join(" ".join(f"{v:3d}" for v in row) for row in self._data)
# 使用
m = Matrix(3, 4)
m[0] = [1, 2, 3, 4]
m[1] = [5, 6, 7, 8]
m[2] = [9, 10, 11, 12]
print(m[0][2]) # 3
print(f"行数: {len(m)}") # 3
print(7 in m) # True
4.2 完整运算符速查表
# 算术运算符 # __add__(+), __sub__(-), __mul__(*), __truediv__(/), __floordiv__(//) # __mod__(%), __pow__(**), __divmod__() # __radd__, __rsub__, ... （反向运算符） # __iadd__, __isub__, ... （增强赋值 += 等） # 比较运算符 # __eq__(==), __ne__(!=), __lt__(<), __le__(<=), __gt__(>), __ge__(>=) # 一元运算符 # __neg__(-), __pos__(+), __abs__(abs()), __invert__(~) # 容器运算符 # __len__(len), __getitem__([]), __setitem__([]=), __delitem__(del []) # __contains__(in), __iter__(for) # 类型转换 # __int__(int), __float__(float), __bool__(bool), __str__(str), __bytes__(bytes) # 其他 # __call__(obj()), __enter__/__exit__(with), __hash__(hash)
五、总结
运算符重载让你的对象用起来像内置类型一样自然。但不要滥用——只在你真的需要、且行为符合直觉时才重载。
核心要点：
- 运算符背后是魔法方法——
+→__add__，==→__eq__ __eq__和__hash__要一起定义——否则set/dict出问题@total_ordering——定义__eq__和__lt__就自动生成全部比较方法- 反向运算用
__radd__等——当左操作数不支持时被调用 - 不支持的操作返回
NotImplemented——让Python尝试对方的反向方法
原则：运算符的行为应该"符合直觉"。+永远是加法或拼接，==永远是比较相等——不要搞创新。
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