简介

海象运算符 :=，又称赋值表达式（Assignment Expression），Python 3.8 后可用，PEP 572 引入，其核心设计是在表达式内部完成变量赋值并返回该值，从而简化代码逻辑。

正常赋值语句是 a=b，读作“a等于b”，而海象赋值语句是 a:=b，读作“a walrus /ˈwɔːlrəs/ b”，因为 := 看起来像海象的眼睛和牙齿。

海象运算符优先级低于比较操作，需用括号明确边界

条件判断优化

import datetimeperson = {'name': 'Alice', 'birthyear': 1997}
currentyear = datetime.datetime.now().yearif birthyear := person.get('birthyear'):print(f'{currentyear - birthyear} years old')# 传统写法
if person.get('birthyear'):birthyear = person['birthyear']  # 调多一次print(f'{currentyear - birthyear} years old')

避免重复计算，提升可读性

循环控制简化

with open('requirements.txt', encoding='utf-8') as f:while line := f.readline():print(line)# 传统写法
with open('requirements.txt', encoding='utf-8') as f:while True:line = f.readline()if not line:breakprint(line)

推导式高效计算

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]squares = [squared for num in numbers if (squared := num ** 2) > 10]  # 计算平方值大于10的结果
print(squares)# 传统写法
squares = [num ** 2 for num in numbers if num ** 2 > 10]
print(squares)

正则匹配与数据提取

import retext = 'Date: 2023-10-05'if (match := re.search(r'(\d{4})-(\d{2})-(\d{2})', text)):year, month, day = match.groups()print(f'{year}-{month}-{day}')# 传统写法
match = re.search(r'(\d{4})-(\d{2})-(\d{2})', text)
if match:year, month, day = match.groups()print(f'{year}-{month}-{day}')

性能对比

import timeit
import random# 10万个随机整数，计算平方值并筛选大于2500的元素
data = [random.randint(1, 100) for _ in range(100000)]def traditional_method():"""传统写法"""return [x ** 2 for x in data if x ** 2 > 2500]def walrus_method():"""海象运算符写法"""return [sq for x in data if (sq := x ** 2) > 2500]traditional_time = timeit.timeit(traditional_method, number=100)
walrus_time = timeit.timeit(walrus_method, number=100)print(f'传统写法耗时: {traditional_time:.4f} 秒')
print(f'海象运算符耗时: {walrus_time:.4f} 秒')
print(f'性能提升: {((traditional_time - walrus_time) / traditional_time) * 100:.2f}%')
# 传统写法耗时: 2.0772 秒
# 海象运算符耗时: 1.5260 秒
# 性能提升: 26.53%