Python 数据模型核心双下划线方法

对象生命周期

示例：

class MyClass:
def __new__(cls, *args, **kwargs):
print(f"__new__ called for {cls}")
return super().__new__(cls)

def __init__(self, value):
print(f"__init__ called with {value}")
self.value = value

def __del__(self):
print(f"__del__ called for {self}")

对象表示

示例：

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

def __str__(self):
return f"Point({self.x}, {self.y})"

def __repr__(self):
return f"Point(x={self.x}, y={self.y})"

def __format__(self, format_spec):
return f"<{self.x}, {self.y}>"

p = Point(3, 4)
print(p) # Point(3, 4) – __str__
print(repr(p) ) # Point(x=3, y=4) – __repr__
print(f"{p}" ) # Point(3, 4) – 默认使用 __str__

比较操作

使用 @functools.total_ordering 简化：

from functools import total_ordering

@total_ordering
class Person:
def __init__(self, age):
self.age = age

def __eq__(self, other):
if not isinstance(other, Person):
return NotImplemented
return self.age == other.age

def __lt__(self, other):
if not isinstance(other, Person):
return NotImplemented
return self.age < other.age

容器类型

容器大小与成员检测

元素访问

示例：自定义列表：

class CustomList:
def __init__(self, items=None):
self._items = items or []

def __len__(self):
return len(self._items)

def __getitem__(self, index):
return self._items[index]

def __setitem__(self, index, value):
self._items[index] = value

def __delitem__(self, index):
del self._items[index]

def __contains__(self, item):
return item in self._items

def __iter__(self):
return iter(self._items)

可调用对象

示例：

class Adder:
def __init__(self, n):
self.n = n

def __call__(self, x):
return self.n + x

add5 = Adder(5)
add5(10) # 返回 15

上下文管理

示例：

class FileContext:
def __init__(self, filename, mode):
self.filename = filename
self.mode = mode

def __enter__(self):
self.file = open(self.filename, self.mode)
return self.file

def __exit__(self, exc_type, exc_val, exc_tb):
self.file.close()
return False # 不抑制异常

with FileContext('test.txt', 'w') as f:
f.write('Hello')

迭代器协议

示例：自定义迭代器：

class CountDown:
def __init__(self, start):
self.current = start

def __iter__(self):
return self

def __next__(self):
if self.current <= 0:
raise StopIteration
self.current -= 1
return self.current + 1

for i in CountDown(5):
print(i) # 输出: 5, 4, 3, 2, 1

数值运算

一元运算

二元运算

位运算

反向运算（右操作数类型不匹配时）

增量赋值

示例：复数实现：

class Complex:
def __init__(self, real, imag):
self.real = real
self.imag = imag

def __add__(self, other):
return Complex(self.real + other.real, self.imag + other.imag)

def __mul__(self, other):
return Complex(
self.real * other.real – self.imag * other.imag,
self.real * other.imag + self.imag * other.real
)

def __repr__(self):
return f"Complex({self.real}, {self.imag})"

类型转换

示例：

class Number:
def __init__(self, value):
self.value = value

def __int__(self):
return int(self.value)

def __float__(self):
return float(self.value)

def __hash__(self):
return hash(self.value)

num = Number(3.14)
int(num) # 3
float(num) # 3.14

属性访问

示例：动态属性：

class DynamicAttrs:
def __init__(self):
self._data = {}

def __getattr__(self, name):
if name in self._data:
return self._data[name]
raise AttributeError(f"'{self.__class__.__name__}' object has no attribute '{name}'")

def __setattr__(self, name, value):
if name.startswith('_'):
super().__setattr__(name, value)
else:
self._data[name] = value

obj = DynamicAttrs()
obj.name = "Alice"
print(obj.name) # Alice

序列协议

示例：

class CountUp:
def __init__(self, max_val):
self.max_val = max_val

def __reversed__(self):
return iter(range(self.max_val, -1, -1))

list(reversed(CountUp(5))) # [5, 4, 3, 2, 1, 0]

核心设计原则

1. NotImplemented vs TypeError

def __eq__(self, other):
if isinstance(other, self.__class__):
return self.value == other.value
return NotImplemented # 让 Python 尝试 other.__eq__(self)

2. 避免无限递归

# ❌ 错误：递归
def __getattr__(self, name):
return self.name # 会再次触发 __getattr__

# ✅ 正确
def __getattr__(self, name):
return self._data[name]

3. __hash__ 与 __eq__ 的一致性

class Person:
def __init__(self, id):
self.id = id

def __eq__(self, other):
return self.id == other.id

def __hash__(self):
return hash(self.id) # 相等的对象必须有相等的 hash

常用组合示例

自定义字典（完整实现）

class CaseInsensitiveDict(dict):
def __setitem__(self, key, value):
super().__setitem__(key.lower(), value)

def __getitem__(self, key):
return super().__getitem__(key.lower())

def __contains__(self, key):
return super().__contains__(key.lower())

d = CaseInsensitiveDict()
d['Name'] = 'Alice'
print(d['name']) # Alice

范围对象（可切片）

class Range:
def __init__(self, start, end=None, step=1):
if end is None:
end = start
start = 0
self.start = start
self.end = end
self.step = step

def __len__(self):
return max(0, (self.end – self.start + self.step – 1) // self.step)

def __getitem__(self, index):
if isinstance(index, slice):
start, stop, step = index.indices(len(self))
return Range(self.start + start * self.step,
self.start + stop * self.step,
self.step * step)
if index < 0:
index += len(self)
if index < 0 or index >= len(self):
raise IndexError("range index out of range")
return self.start + index * self.step

def __iter__(self):
return iter(self[i] for i in range(len(self)))

list(Range(10)[2:7:2]) # [2, 4, 6]

实践建议

参考资料

• Python Data Model – Python 官方文档
• Python Tricks: The Book – Dan Bader