Python 条件语句完全指南：if、else、elif 与字典模拟 switch-case

Python 条件语句是什么？

条件语句让程序基于布尔表达式执行不同代码块。Python 中 if 语句是驱动决策的核心构造。

本指南探索 if、else、elif 及字典模拟 switch-case，每节含简洁示例。

理解 if 语句
if 条件为假时的行为
使用 else 分支
else 常见陷阱
用 elif 处理多分支
三元表达式简化逻辑
嵌套 if 语句
字典实现 switch-case 模式

Python `if` 语句基础

语法

if expression:
    # 表达式为 True 执行
else:
    # 表达式为 False 执行

示例：

def main():
    x, y = 2, 8
    if x < y:
        st = "x is less than y"
    else:
        st = "x is not less than y"
    print(st)

if __name__ == "__main__":
    main()

`if` 条件为假时会发生什么？

条件为 False 且无 else 时，if 体被跳过，程序继续执行后续代码。

条件失败示例：

def main():
    x, y = 8, 4
    if x < y:
        st = "x is less than y"
    else:
        st = "x is greater than or equal to y"
    print(st)

if __name__ == "__main__":
    main()

使用 `else` 分支

else 块在前置 if 为 False 时执行，适合二元决策。

def main():
    x, y = 8, 4
    if x < y:
        st = "x is less than y"
    else:
        st = "x is greater than y"
    print(st)

if __name__ == "__main__":
    main()

`else` 常见错误

两值相等时，else 可能错误报告“greater”或“less”。

def main():
    x, y = 8, 8
    if x < y:
        st = "x is less than y"
    else:
        st = "x is greater than y"  # x==y 时错误
    print(st)

if __name__ == "__main__":
    main()

引入 `elif` 处理多分支

elif 允许在初始 if 后测试额外条件，避免上述落穿错误。

def main():
    x, y = 8, 8
    if x < y:
        st = "x is less than y"
    elif x == y:
        st = "x is the same as y"
    else:
        st = "x is greater than y"
    print(st)

if __name__ == "__main__":
    main()

三元表达式简化逻辑

Python 单行语法让简单比较更简洁：

def main():
    x, y = 10, 8
    st = "x is less than y" if x < y else "x is greater than or equal to y"
    print(st)

if __name__ == "__main__":
    main()

嵌套 `if` 语句

多变量依赖决策时用嵌套条件。

total = 100
country = "AU"  # 改为 "US" 测试另一分支
if country == "US":
    if total <= 50:
        print("Shipping Cost is $50")
    elif total <= 100:
        print("Shipping Cost is $25")
    elif total <= 150:
        print("Shipping Costs $5")
    else:
        print("FREE")
elif country == "AU":
    if total <= 50:
        print("Shipping Cost is $100")
    else:
        print("FREE")

Python 中的 Switch-Case 替代方案

Python 无原生 switch，但字典查找提供同等清晰度与效率。

def switch_example(argument):
    switcher = {
        0: "This is Case Zero",
        1: "This is Case One",
        2: "This is Case Two",
    }
    return switcher.get(argument, "nothing")

if __name__ == "__main__":
    argument = 1
    print(switch_example(argument))