Python序列解包

序列解包将序列元素分解到多个变量，支持扩展解包和字典解包。

基本解包

元组解包

coordinates = (10, 20)
x, y = coordinates
print(x, y)  # 10 20

列表解包

values = [1, 2, 3]
a, b, c = values
print(a, b, c)  # 1 2 3

字符串解包

s = "abc"
x, y, z = s
print(x, y, z)  # a b c

变量交换

a, b = 10, 20
a, b = b, a
print(a, b)  # 20 10

扩展解包（*）

收集剩余元素

values = [1, 2, 3, 4, 5]

first, *rest = values
print(first)  # 1
print(rest)   # [2, 3, 4, 5]

*head, last = values
print(head)   # [1, 2, 3, 4]
print(last)   # 5

first, *middle, last = values
print(first)    # 1
print(middle)   # [2, 3, 4]
print(last)     # 5

忽略中间元素

values = [1, 2, 3, 4, 5]
first, _, _, _, last = values  # 用 _ 忽略
print(first, last)  # 1 5

# 使用扩展解包更简洁
first, *_, last = values
print(first, last)  # 1 5

分割列表

data = [1, 2, 3, 4, 5, 6]
left, *right = data
print(left)   # 1
print(right)  # [2, 3, 4, 5, 6]

字典解包（**）

函数调用解包

def greet(name, age, city):
    print(f"{name}, {age}, {city}")

params = {"name": "Alice", "age": 25, "city": "Beijing"}
greet(**params)  # Alice, 25, Beijing

# 等价于
greet(name="Alice", age=25, city="Beijing")

合并字典

dict1 = {"a": 1, "b": 2}
dict2 = {"c": 3, "d": 4}

# Python 3.9+ 合并运算符
merged1 = dict1 | dict2

# Python 3.5+ 解包合并
merged2 = {**dict1, **dict2}
print(merged2)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

带默认值合并

defaults = {"host": "localhost", "port": 3306}
user_config = {"port": 3307}

# 用户配置覆盖默认值
config = {**defaults, **user_config}
print(config)  # {'host': 'localhost', 'port': 3307}

函数参数解包

列表解包为位置参数

def add(a, b, c):
    return a + b + c

values = [1, 2, 3]
print(add(*values))  # 6

元组解包为位置参数

def power(base, exponent):
    return base ** exponent

params = (2, 3)
print(power(*params))  # 8

实际应用

循环解包

# 解包嵌套序列
pairs = [(1, "a"), (2, "b"), (3, "c")]
for num, letter in pairs:
    print(f"{num}: {letter}")

# 1: a
# 2: b
# 3: c

字典遍历解包

d = {"a": 1, "b": 2, "c": 3}
for key, value in d.items():
    print(f"{key} = {value}")

函数返回多值解包

def get_info():
    return "Alice", 25, "Beijing"

name, age, city = get_info()
print(name, age, city)  # Alice 25 Beijing

文件读取解包

# CSV 行解包
line = "Alice,25,Beijing"
name, age, city = line.split(",")
print(name, age, city)  # Alice 25 Beijing

解包注意事项

元素数量必须匹配

values = [1, 2, 3]
# a, b = values    # ValueError：太多元素
# a, b, c, d = values  # ValueError：元素不足

# 使用 * 解决
a, b, *rest = values
print(a, b, rest)  # 1 2 [3]

* 只能出现一次

values = [1, 2, 3, 4, 5]
# *a, *b, c = values  # SyntaxError

# 正确用法
a, *b, c = values
print(a, b, c)  # 1 [2, 3, 4] 5

解包类型对比

要点总结

1. 基本解包要求元素数量与变量数量匹配
2. • 收集剩余元素为列表，只能使用一次
3. ** 解包字典为关键字参数或合并字典
4. 常用于变量交换、循环遍历、函数调用
5. 使用 _ 忽略不需要的元素