Python流程控制与函数

流程控制与函数

条件语句

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"

# 三元表达式
grade = "A" if score >= 90 else "B"

# match-case（Python 3.10+）
match status:
    case 200:
        print("OK")
    case 404:
        print("Not Found")
    case _:
        print("Unknown")

循环

for 循环

1
2
3
4
5
6
7
8
9
10
11
12
for i in range(5):       # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 2):  # 2, 4, 6, 8
    print(i)

fruits = ["apple", "banana"]
for fruit in fruits:
    print(fruit)

for idx, fruit in enumerate(fruits):
    print(idx, fruit)

while 循环

1
2
3
4
n = 0
while n < 5:
    print(n)
    n += 1

break 与 continue

1
2
3
4
5
6
for i in range(10):
    if i == 3:
        continue   # 跳过本次
    if i == 7:
        break      # 退出循环
    print(i)

else 子句

循环正常结束（未 break）时执行 else：

1
2
3
4
5
6
for x in data:
    if x < 0:
        print("found negative")
        break
else:
    print("all positive")

函数定义

1
2
3
4
5
6
7
8
9
10
11
def add(a: int, b: int) -> int:
    """返回 a + b。"""
    return a + b

# 默认参数（默认值在定义时求值，可变默认值是常见陷阱）
def greet(name: str, prefix: str = "Hello") -> str:
    return f"{prefix}, {name}!"

# 关键字参数
greet(name="Alice", prefix="Hi")
greet("Bob")  # 位置参数

可变参数

1
2
3
4
5
6
7
8
9
10
def sum_all(*args: int) -> int:
    return sum(args)

sum_all(1, 2, 3, 4)  # 10

def print_info(**kwargs) -> None:
    for k, v in kwargs.items():
        print(f"{k}={v}")

print_info(name="Alice", age=25)

仅关键字参数

1
2
3
4
def connect(host: str, *, port: int = 5432, timeout: int = 30):
    ...

connect("localhost", port=8080)  # host 位置，port/timeout 必须关键字

Lambda 表达式

匿名函数，适合简单场景：

1
2
square = lambda x: x ** 2
sorted(items, key=lambda x: x["score"], reverse=True)

复杂逻辑应使用 def，lambda 仅一行表达式。

作用域 LEGB

查找顺序：Local → Enclosing → Global → Built-in

1
2
3
4
5
6
7
8
9
10
x = "global"

def outer():
    x = "enclosing"
    def inner():
        x = "local"
        print(x)
    inner()

outer()  # local

修改外层变量用 nonlocal，修改全局用 global（尽量少用）：

1
2
3
4
5
6
7
def counter():
    count = 0
    def inc():
        nonlocal count
        count += 1
        return count
    return inc

推导式

1
2
3
4
5
6
7
8
9
10
11
12
# 列表推导
squares = [x ** 2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]

# 字典推导
{d: d ** 2 for d in range(5)}

# 集合推导
{x % 3 for x in range(10)}

# 生成器表达式（惰性，省内存）
sum(x ** 2 for x in range(1000000))

小结

  • if/elif/else、for、while 构成流程控制主干
  • 函数支持默认参数、*args、**kwargs
  • 避免可变默认参数 [ ] 或 {}，改用 None
  • 列表推导式简洁，但复杂逻辑仍用普通循环