Python3 条件控制详解：从基础语法到高级应用详解

if condition:
    # 当 condition 为 True 时执行的代码块
    statement1
    statement2
    ...

age = 18
if age >= 18:
    print("成年人")  # 输出：成年人

if condition:
    # 条件为真时执行的语句块
else:
    # 条件为假时执行的语句块

age = 16
if age >= 18:
    print("成年人")
else:
    print("未成年人")  # 输出：未成年人

if condition1:
    # 条件1为真时执行的语句块
elif condition2:
    # 条件1为假，条件2为真时执行的语句块
elif condition3:
    # 条件1,2为假，条件3为真时执行的语句块
...
else:
    # 上述所有条件都为假时执行的语句块

score = 85
if score >= 90:
    print("优秀")
elif score >= 80:
    print("良好")  # 输出：良好
elif score >= 70:
    print("中等")
elif score >= 60:
    print("及格")
else:
    print("不及格")

if condition1:
    if condition2:
        # 条件1和条件2都为真时执行的语句块
    else:
        # 条件1为真但条件2为假时执行的语句块
else:
    # 条件1为假时执行的语句块

num = 10
if num > 0:
    if num % 2 == 0:
        print("正偶数")  # 输出：正偶数
    else:
        print("正奇数")
else:
    if num == 0:
        print("零")
    else:
        print("负数")

def check_number(num):
    if num <= 0:
        if num == 0:
           return "零"
        else:
            return "负数"
    # 主逻辑：处理正数
    if num % 2 == 0:
        return "正偶数"
    else:
        return "正奇数"

year = 2024
if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
    print(f"{year} 是闰年")  # 输出：2024 是闰年
else:
    print(f"{year} 不是闰年")

# 假设我们不确定 user 是否为 None
# 较安全的写法：如果 user 存在，才检查其是否活跃
if user is not None and user.is_active:
    print("用户在线")

value_if_true if condition else value_if_false

age = 20
status = "成年" if age >= 18 else "未成年"
print(f"状态: {status}")  # 输出：状态: 成年

if age >= 18:
    status = "成年"
else:
    status = "未成年"

match expression:
    case pattern1:
        # 当 expression 与 pattern1 匹配时执行的代码块
        ...
    case pattern2:
        # 当 expression 与 pattern2 匹配时执行的代码块
        ...
    case _:
        # 默认情况，当 expression 与所有前面的模式都不匹配时执行
        ...

def http_error(status):
    match status:
        case 400:
            return "Bad request"
        case 404:
            return "Not found"
        case 418:
            return "I'm a teapot"
        case _:
            return "Something's wrong with the Internet"
print(http_error(404))  # 输出：Not found

match status:
    case 401 | 403 | 404:
        return "Not allowed"

def process_point(point):
    match point:
        case (0, 0):
            print("原点")
        case (0, y):
            print(f"点在Y轴上，Y={y}")
        case (x, 0):
            print(f"点在X轴上，X={x}")
        case (x, y):
            print(f"点坐标为 ({x}, {y})")
        case _:
            print("不是坐标点")
process_point((3, 5))  # 输出：点坐标为 (3, 5)

class Pet:
    def __init__(self, name, species):
        self.name = name
        self.species = species
def describe_pet(pet):
    match pet:
        case Pet(name=name, species="dog"):
            print(f"有一只名叫{name}的狗")
        case Pet(name=name, species="cat"):
            print(f"有一只名叫{name}的猫")
        case _:
            print("未知宠物")
describe_pet(Pet("旺财", "dog"))  # 输出：有一只名叫旺财的狗

score = 85
if score >= 90:
    print("优秀")
if score >= 80:  # 这个也会执行！
    print("良好")

def categorize_age(age):
    if age < 0:
        print("年龄不能为负数")
    elif age < 13:
        print("儿童")
    elif age < 18:
        print("青少年")
    elif age < 65:
        print("成年人")
    else:
        print("老年人")

items = [1, 2, 3]
if items:  # 等价于 if len(items) > 0:
    print(f"列表中有 {len(items)} 个元素")
else:
    print("列表为空")

if (user.age >= 18 and user.is_verified) or (user.has_special_permit and not user.is_banned):
    allow_access()

is_adult_verified = user.age >= 18 and user.is_verified
has_exception = user.has_special_permit and not user.is_banned
if is_adult_verified or has_exception:
    allow_access()

def can_access(user):
    return (user.age >= 18 and user.is_verified) or (user.has_special_permit and not user.is_banned)
if can_access(user):
    allow_access()

def safe_divide():
    """安全地进行除法运算，处理各种输入错误"""
    try:
        dividend = float(input("请输入被除数："))
        divisor = float(input("请输入除数："))
    except ValueError:
        print("错误：输入必须是数字。")
        return
    if divisor == 0:
        print("错误：除数不能为零。")
        return
    result = dividend / divisor
    print(f"结果是：{result}")

def process_data(data_list):
    """根据列表内容的不同处理数据"""
    if not data_list:  # 空列表检测
        print("数据为空")
        return
    if all(isinstance(item, int) for item in data_list):
        print("所有数据项都是整数")
        print("数据的总和是：", sum(data_list))
    elif all(isinstance(item, str) for item in data_list):
        print("所有数据项都是字符串")
        print("数据拼接结果是：", " ".join(data_list))
    else:
        print("数据包含不同类型，无法统一处理。")

def control_flow_example(state):
    """一个简单的状态机示例"""
    match state:  # 使用 match...case 更清晰地表达状态转移
        case "start":
            print("程序开始")
        case "process":
            print("程序处理中")
        case "end":
            print("程序结束")
        case _:
            print("未知状态")
states = ["start", "process", "end", "unknown"]
for s in states:
    control_flow_example(s)

def simple_calculator():
    print("简单计算器 (支持 +, -, *, /)")
    try:
        num1 = float(input("输入第一个数字: "))
        op = input("输入运算符 (+, -, *, /): ")
        num2 = float(input("输入第二个数字: "))
    except ValueError:
        print("数字输入无效。")
        return
    match op:
        case '+':
            result = num1 + num2
        case '-':
            result = num1 - num2
        case '*':
            result = num1 * num2
        case '/':
            if num2 == 0:
                print("错误：除数不能为0。")
                return
            result = num1 / num2
        case _:
            print("无效的运算符。")
            return
    print(f"{num1} {op} {num2} = {result}")
# 运行计算器
simple_calculator()