# 找出第一个负数
for n in [3, 5, -1, 7, -2]:
    if n < 0:
        print(f"找到 {n}")
        break     # 立刻退出，后面 7 和 -2 不再看

# 只打印偶数
for n in range(10):
    if n % 2 != 0:
        continue
    print(n)

# 用 continue（Guard 式）
for n in numbers:
    if not is_valid(n):
        continue
    process(n)     # 只有合法元素才处理

# 不用 continue
for n in numbers:
    if is_valid(n):
        process(n)

for order in orders:
    if not order.paid:
        continue
    if order.refunded:
        continue
    if not order.address:
        log("no address")
        continue
    if order.amount <= 0:
        continue
    # 到这里说明这是一个可以处理的订单
    ship(order)

for n in [3, 5, 7, 11]:
    if n % 2 == 0:
        print("发现偶数")
        break
else:
    print("全都是奇数")     # 只有循环跑完（没 break）才执行

def find(items, target):
    for i, x in enumerate(items):
        if x == target:
            print(f"找到 {target} 在索引 {i}")
            break
    else:
        print(f"{target} 不存在")

found = False
for i, x in enumerate(items):
    if x == target:
        found = True
        break
if not found:
    print("不存在")

attempts = 0
while attempts < 3:
    pwd = input("密码：")
    if pwd == "123456":
        print("登录成功")
        break
    attempts += 1
else:
    print("三次输错，账户已锁定")

def find(matrix, target):
    for row in matrix:
        for cell in row:
            if cell == target:
                return True
    return False

if find(m, 42):
    print("找到")

found = False
for row in matrix:
    for cell in row:
        if cell == target:
            found = True
            break
    if found:
        break

class _Found(Exception):
    pass

try:
    for row in matrix:
        for cell in row:
            if cell == target:
                raise _Found
except _Found:
    print("找到")

from itertools import product
for row, cell in product(matrix, matrix[0]):
    ...

i = 0
while i < 10:
    if i == 5:
        continue    # ❌ i 没变，永远卡在 5！
    print(i)
    i += 1

i = 0
while i < 10:
    if i == 5:
        i += 1
        continue
    print(i)
    i += 1

# login.py
"""控制台登录模拟，演示循环控制三件套。"""

USERS = {
    "admin": "admin888",
    "alice": "alice123",
    "bob":   "bob@2025",
}

MAX_ATTEMPTS = 3


def login() -> bool:
    for attempt in range(1, MAX_ATTEMPTS + 1):
        user = input(f"用户名（第 {attempt} 次）：").strip()
        if not user:
            print("用户名不能为空")
            continue     # 空的直接下一轮，不占尝试次数？—— 这里演示占用

        if user not in USERS:
            print("用户不存在")
            continue

        pwd = input("密码：")
        if pwd == USERS[user]:
            print(f"欢迎，{user}")
            return True
        else:
            print(f"密码错误，还剩 {MAX_ATTEMPTS - attempt} 次机会")
    else:
        # 循环正常跑完（没 return / break）→ 三次都错
        print("三次尝试失败，账户已锁定 5 分钟")
        return False


def command_loop():
    print("输入命令（quit 退出）：")
    while True:
        cmd = input("> ").strip()
        if not cmd:
            continue          # 空行忽略
        if cmd in {"quit", "exit"}:
            print("再见")
            break
        if cmd == "help":
            print("可用命令：help, status, quit")
            continue
        print(f"未识别命令 {cmd!r}")


if __name__ == "__main__":
    if login():
        command_loop()

def find(items, target):
    for i, x in enumerate(items):
        if x == target:
            return i    # ✅ 直接返回，比 break 后 return 简洁
    return -1

for x in items:
    print(x)
else:
    print("done")    # 一定会执行，跟没 else 一样

for x in items:
    if x < 0:
        continue
        print("负数")    # 永远执行不到
    print(x)

def outer():
    for x in items:
        def inner():
            for y in items:
                break     # 只跳出 inner 的循环，不影响 outer
        inner()

while True:
    cmd = input()
    print(cmd)   # 永远不退出

for 变量 in 可迭代对象:
    循环体

for name in ["咖飞", "小王", "小李"]:
    print(f"你好，{name}")

for ch in "Python":
    print(ch)

for key in {"a": 1, "b": 2}:      # 遍历字典默认遍历 key
    print(key)

for i in range(5):
    print(i)
# 0 1 2 3 4

for i in range(1, 6):     # [1, 6)
    print(i)
# 1 2 3 4 5

for i in range(0, 10, 2): # start=0, stop=10, step=2
    print(i)
# 0 2 4 6 8

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

names = ["咖飞", "小王", "小李"]
for i in range(len(names)):
    print(i, names[i])

for i, name in enumerate(names):
    print(i, name)

for i, name in enumerate(names, start=1):
    print(f"第 {i} 名：{name}")

names = ["咖飞", "小王"]
ages = [3, 20]

for name, age in zip(names, ages):
    print(f"{name}，{age} 岁")

list(zip([1, 2, 3], ["a", "b"]))    # [(1, 'a'), (2, 'b')]

score = {"张三": 90, "李四": 85, "王五": 78}

# 只要 key
for name in score:
    print(name)

# 要 key 也要 value
for name, s in score.items():
    print(name, s)

# 只要 value
for s in score.values():
    print(s)

count = 0
while count < 5:
    print(count)
    count += 1

while True:
    request = get_next_request()
    if request is None:
        break
    handle(request)

while (line := input()):     # 海象运算符
    process(line)

# for：处理一批固定数据
for user in users:
    send_email(user)

# while：一直读，读到"quit"退出
while True:
    cmd = input()
    if cmd == "quit":
        break
    handle(cmd)

# 找第一个偶数
for n in [1, 3, 5, 4, 7]:
    if n % 2 == 0:
        print(f"找到 {n}")
        break
else:
    print("没找到")

# continue：跳过负数
for n in [3, -1, 5, -2, 8]:
    if n < 0:
        continue
    print(n)

# 判断质数
n = 17
for i in range(2, n):
    if n % i == 0:
        print(f"{n} 不是质数")
        break
else:
    print(f"{n} 是质数")

# 九九乘法表
for i in range(1, 10):
    for j in range(1, i + 1):
        print(f"{j}*{i}={i*j}", end="\t")
    print()

# 方式 1：用 flag
found = False
for row in matrix:
    for cell in row:
        if cell == target:
            found = True
            break
    if found:
        break

# 方式 2：抽成函数，用 return
def find(matrix, target):
    for row in matrix:
        for cell in row:
            if cell == target:
                return True
    return False

# ❌ 每次循环都调用 len()
for i in range(len(a)):
    ...

# ✅ 更好的是能不用索引就不用
for x in a:
    ...

# ❌ O(n)
def contains(items, target):
    for x in items:
        if x == target:
            return True
    return False

# ✅ 一步搞定
target in items

# ❌ 生成 100 万个中间元素
sum([x*x for x in range(10**6)])

# ✅ 生成器表达式，边算边加
sum(x*x for x in range(10**6))

total = 0
for x in numbers:
    total += x
# 等价：total = sum(numbers)

result = []
for x in items:
    if x > 0:
        result.append(x * 2)
# 等价：result = [x*2 for x in items if x > 0]（列表推导式，第 15 篇讲）

counter = {}
for w in words:
    counter[w] = counter.get(w, 0) + 1
# 等价：from collections import Counter; Counter(words)

def find_first_negative(nums):
    for n in nums:
        if n < 0:
            return n
    return None

# primes.py
"""筛选出 2..N 之间的所有质数（埃拉托色尼筛法）。"""

def sieve(n: int) -> list[int]:
    if n < 2:
        return []
    is_prime = [True] * (n + 1)
    is_prime[0] = is_prime[1] = False

    for i in range(2, int(n ** 0.5) + 1):
        if is_prime[i]:
            # 从 i*i 开始把 i 的倍数标为合数
            for j in range(i * i, n + 1, i):
                is_prime[j] = False

    return [i for i, prime in enumerate(is_prime) if prime]


def main():
    n = int(input("求 2 到 N 之间的所有质数，请输入 N："))
    primes = sieve(n)
    print(f"共 {len(primes)} 个质数：")
    # 每行 10 个
    for i, p in enumerate(primes):
        print(f"{p:5d}", end="\n" if (i + 1) % 10 == 0 else " ")
    print()


if __name__ == "__main__":
    main()

items = [1, 2, 3, 4, 5]
for x in items:
    if x % 2 == 0:
        items.remove(x)   # 危险！索引错乱
print(items)              # 可能不是你想的

items = [x for x in items if x % 2]   # 保留奇数
# 或者：for x in items[:]:   遍历副本

for i in range(3):
    pass
print(i)     # 输出 2，i 泄漏到外层作用域

d = {"a": 1, "b": 2}
for k in d:
    if d[k] == 1:
        del d[k]    # RuntimeError: dictionary changed size

n = input("次数：")
for i in range(n):      # TypeError，n 是字符串
    ...

age = int(input("年龄："))

if age >= 18:
    print("成年")

age = int(input("年龄："))

if age >= 18:
    print("成年")
else:
    print("未成年")

score = int(input("分数："))

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"等级 {grade}")

name = input("名字：")

if name:                    # ✅ Pythonic
    print(f"你好，{name}")
else:
    print("你没输入名字")

# ❌ 冗长
if name != "":
    print(...)

age = 25
has_ticket = True
is_vip = False

if age >= 18 and has_ticket:
    print("可以进场")

if is_vip or age >= 65:
    print("免排队")

if not has_ticket:
    print("买票先")

age = 25

# 好，Python 特有
if 18 <= age < 60:
    print("劳动年龄")

# 不好
if age >= 18 and age < 60:
    print("劳动年龄")

age = int(input("年龄："))
has_id = input("有身份证吗？(y/n)：") == "y"

if age >= 18:
    if has_id:
        print("可以办卡")
    else:
        print("请先办身份证")
else:
    print("年龄不足")

# 更清爽
if age < 18:
    print("年龄不足")
elif not has_id:
    print("请先办身份证")
else:
    print("可以办卡")

def apply_card(age: int, has_id: bool) -> str:
    if age < 18:
        return "年龄不足"
    if not has_id:
        return "请先办身份证"
    return "可以办卡"

label = "成年" if age >= 18 else "未成年"

# 常用于生成列表元素
tags = ["young" if a < 18 else "adult" for a in ages]

def http_status(code: int) -> str:
    match code:
        case 200:
            return "OK"
        case 301 | 302:                    # 或匹配
            return "Redirect"
        case 400 | 401 | 403 | 404:
            return "Client Error"
        case 500:
            return "Server Error"
        case _:                             # 通配，等价于 default
            return "Unknown"

def describe_point(point: tuple) -> str:
    match point:
        case (0, 0):
            return "原点"
        case (0, y):
            return f"Y 轴上，y={y}"
        case (x, 0):
            return f"X 轴上，x={x}"
        case (x, y):
            return f"点 ({x}, {y})"
        case _:
            return "不是坐标"

print(describe_point((0, 0)))     # 原点
print(describe_point((5, 0)))     # X 轴上，x=5
print(describe_point((3, 4)))     # 点 (3, 4)

def handle_event(event: dict) -> str:
    match event:
        case {"type": "click", "x": x, "y": y}:
            return f"点击 ({x}, {y})"
        case {"type": "keydown", "key": key}:
            return f"按键 {key}"
        case {"type": t}:
            return f"未知事件 {t}"
        case _:
            return "格式不对"

print(handle_event({"type": "click", "x": 10, "y": 20}))   # 点击 (10, 20)
print(handle_event({"type": "keydown", "key": "Enter"}))   # 按键 Enter

def category(age: int) -> str:
    match age:
        case n if n < 0:
            return "非法"
        case n if n < 18:
            return "未成年"
        case n if n < 60:
            return "成年"
        case _:
            return "老人"

# rps.py
"""石头剪刀布，展示 if 与 match 的组合。"""

import random


def get_computer_choice() -> str:
    return random.choice(["石头", "剪刀", "布"])


def get_user_choice() -> str:
    while True:
        raw = input("请出招（石头/剪刀/布/quit）：").strip()
        if raw == "quit":
            return raw
        if raw in {"石头", "剪刀", "布"}:
            return raw
        print("请输入正确选项")


def judge(user: str, computer: str) -> str:
    if user == computer:
        return "平局"
    # 用 match 表达"什么克什么"
    match user:
        case "石头":
            return "赢" if computer == "剪刀" else "输"
        case "剪刀":
            return "赢" if computer == "布" else "输"
        case "布":
            return "赢" if computer == "石头" else "输"
        case _:
            return "非法"


def main():
    score = {"赢": 0, "输": 0, "平局": 0}
    while True:
        user = get_user_choice()
        if user == "quit":
            break
        computer = get_computer_choice()
        result = judge(user, computer)
        score[result] += 1
        print(f"电脑：{computer}，结果：{result}")

    print(f"总战绩：{score}")


if __name__ == "__main__":
    main()

FOO = 1
def f(x):
    match x:
        case FOO:      # 陷阱！FOO 被当成新变量，永远匹配
            return "matched"

from enum import Enum
class Color(Enum):
    RED = 1

def f(c):
    match c:
        case Color.RED:    # 带点号的名字才被识别为常量匹配
            return "red"

print("Hello, World!")
print("咖飞", "3 岁")             # 多个参数，默认用空格分隔
print("咖飞", "3 岁", sep=" - ")   # 自定义分隔符
print("行 1", end=" | ")           # 不换行，用 " | " 结束
print("行 2")                      # 输出：行 1 | 行 2

with open("log.txt", "w", encoding="utf-8") as f:
    print("日志内容", file=f)

import sys
print("这是错误信息", file=sys.stderr)

import time

for i in range(5):
    print(f"进度 {i}", end=" ")
    time.sleep(1)
# 5 秒后一次性打印，因为没换行触发刷新

for i in range(5):
    print(f"进度 {i}", end=" ", flush=True)
    time.sleep(1)

print([1, 2, 3])         # [1, 2, 3]
print({"a": 1, "b": 2})  # {'a': 1, 'b': 2}
print((1, 2))            # (1, 2)
print(None)              # None

s = "咖飞"
print(s)          # 咖飞
print(repr(s))    # '咖飞'  带引号，能看出是字符串

from pprint import pprint
data = {"users": [{"name": "a"}, {"name": "b"}]}
pprint(data, width=40)

name = input("请输入你的名字：")
print(f"你好，{name}")

age = int(input("年龄："))         # 输入 abc 会抛 ValueError
weight = float(input("体重："))

def read_int(prompt: str) -> int:
    """反复让用户输入直到输入合法整数。"""
    while True:
        raw = input(prompt).strip()
        try:
            return int(raw)
        except ValueError:
            print("请输入合法整数")

age = read_int("年龄：")

raw = input("请输入身高体重(用空格分隔)：")   # 例：170 65
parts = raw.split()
if len(parts) != 2:
    print("格式错误")
else:
    height, weight = map(float, parts)
    print(f"BMI: {weight / (height/100)**2:.2f}")

from getpass import getpass
pwd = getpass("请输入密码：")   # 输入时不显示

name = "咖飞"
age = 3
print(f"我叫 {name}，今年 {age} 岁")

# 表达式也可以
print(f"明年 {age + 1} 岁")
print(f"名字长度 {len(name)}")

# 调用方法
print(f"大写：{name.upper()}")

pi = 3.14159265
print(f"{pi:.2f}")          # 3.14      保留 2 位小数
print(f"{pi:10.2f}")        # '      3.14'  总宽度 10，右对齐
print(f"{pi:<10.2f}")       # '3.14      '  左对齐
print(f"{pi:^10.2f}")       # '   3.14   '  居中
print(f"{pi:>10.2f}")       # '      3.14'  右对齐（默认）
print(f"{pi:0>10.2f}")      # '0000003.14'  填充 0

big = 1234567890
print(f"{big:,}")           # 1,234,567,890  千分位分隔
print(f"{big:_}")           # 1_234_567_890  下划线分隔

rate = 0.256
print(f"{rate:.1%}")        # 25.6%     自动 *100 加 %

n = 255
print(f"{n:b}")             # 11111111    二进制
print(f"{n:o}")             # 377         八进制
print(f"{n:x}")             # ff          十六进制小写
print(f"{n:X}")             # FF          十六进制大写
print(f"{n:#x}")            # 0xff        带前缀
print(f"{n:08b}")           # 11111111    补零到 8 位

from datetime import datetime
now = datetime.now()
print(f"{now:%Y-%m-%d %H:%M:%S}")     # 2025-09-05 09:12:38

x = 42
y = 10
print(f"{x=}, {y=}, {x*y=}")
# x=42, y=10, x*y=420

name = "咖飞"
report = f"""
姓名：{name}
年龄：{3}
状态：{"在学习中" if True else "偷懒"}
"""
print(report)

print(f"{{}}")       # {}
print(f"字典字面：{{'a': 1}}")   # 字典字面：{'a': 1}

name, age = "咖飞", 3

# 第 1 代：% (C printf 风格，Python 2 时代)
print("我叫 %s，%d 岁" % (name, age))

# 第 2 代：str.format() (Python 2.6+)
print("我叫 {}，{} 岁".format(name, age))
print("我叫 {n}，{a} 岁".format(n=name, a=age))

# 第 3 代：f-string (Python 3.6+)  ← 推荐
print(f"我叫 {name}，{age} 岁")

# 从配置里读的模板
template = "Hello, {name}! Today is {date}"
print(template.format(name="咖飞", date="2025-09-05"))

# atm.py
"""极简 ATM 提款机演示 input/print/f-string 用法。"""

from getpass import getpass


def prompt_amount() -> float:
    while True:
        raw = input("请输入提款金额（元）：").strip()
        try:
            amount = float(raw)
        except ValueError:
            print("请输入数字")
            continue
        if amount <= 0:
            print("金额必须大于 0")
            continue
        if amount % 100 != 0:
            print("金额必须是 100 的整数倍")
            continue
        return amount


def main():
    balance = 8888.88
    pwd = getpass("请输入 6 位密码：")
    if pwd != "123456":
        print("密码错误，卡已吞", flush=True)
        return

    print(f"欢迎，当前余额：¥{balance:,.2f}")
    amount = prompt_amount()
    if amount > balance:
        print(f"余额不足！你想取 ¥{amount:,.2f}，但只有 ¥{balance:,.2f}")
        return

    balance -= amount
    print("=" * 30)
    print(f"取款成功：¥{amount:,.2f}")
    print(f"当前余额：¥{balance:,.2f}")
    print(f"时间：{__import__('datetime').datetime.now():%Y-%m-%d %H:%M:%S}")
    print("=" * 30)


if __name__ == "__main__":
    main()

# hello.py
import sys
print(f"参数：{sys.argv}")

s1 = 'hello'          # 单引号
s2 = "hello"          # 双引号，与单引号完全等价
s3 = '''多行
字符串'''              # 三引号，可跨行
s4 = """多行
字符串"""

# 里面含有引号
q1 = "He said 'hi'"
q2 = 'It\'s ok'       # 用反斜杠转义
q3 = "It's ok"        # 或者混用引号

print("换行\n再来一行")
print("\t缩进")            # 制表符
print("反斜杠 \\")
print("Unicode: 中文")   # 中文
print("原生字符串 r'C:\Users\name'")   # 会转义 \U
print(r"原生字符串 r'C:\Users\name'")   # 不转义，Windows 路径必备

path = r"C:\Users\name\file.txt"       # 推荐
path = "C:\\Users\\name\\file.txt"     # 也行
path = "C:/Users/name/file.txt"        # 也行

s = "hello"
s[0] = "H"       # TypeError!

s = "H" + s[1:]        # "Hello"
s = s.replace("h", "H")
s = s.upper()

s = "Python"
print(s[0])        # P，从 0 开始
print(s[-1])       # n，负数索引从右往左
print(len(s))      # 6

print(s[0:2])      # Py，切片 [start:stop)，不含 stop
print(s[2:])       # thon，省略 stop 到结尾
print(s[:3])       # Pyt，省略 start 从头开始
print(s[::-1])     # nohtyP，反转字符串
print(s[::2])      # Pto，步长为 2

for ch in "咖飞":
    print(ch)
# 咖
# 飞

# 需要索引时：
for i, ch in enumerate("Python"):
    print(i, ch)

s = "Hello, Python"

print(s.upper())      # HELLO, PYTHON
print(s.lower())      # hello, python
print(s.title())      # Hello, Python  每个词首字母大写
print(s.capitalize()) # Hello, python  首字母大写，其余小写
print(s.swapcase())   # hELLO, pYTHON  大小写互换

s = "hello world"

print(s.find("o"))         # 4      第一次出现的位置，找不到返回 -1
print(s.rfind("o"))        # 7      从右往左找
print(s.index("o"))        # 4      同 find，但找不到抛 ValueError
print(s.count("l"))        # 3      出现次数

print(s.startswith("he"))  # True
print(s.endswith("ld"))    # True

print("abc123".isalnum())    # True，全是字母数字
print("abc".isalpha())       # True，全是字母
print("123".isdigit())       # True，全是数字
print("   ".isspace())       # True，全是空白
print("ABC".isupper())       # True
print("abc".islower())       # True
print("Hello World".istitle()) # True

s = "   hello   "
print(repr(s.strip()))      # 'hello'  去两端空白
print(repr(s.lstrip()))     # 'hello   '
print(repr(s.rstrip()))     # '   hello'

# 也可以去除指定字符
print("__abc__".strip("_"))   # abc
print("www.baidu.com".lstrip("w."))    # baidu.com

# Python 3.9+ 有更精确的 removeprefix / removesuffix
print("test_module.py".removesuffix(".py"))   # test_module
print("Mr. Wang".removeprefix("Mr. "))         # Wang

s = "hello world hello"
print(s.replace("hello", "hi"))            # hi world hi
print(s.replace("hello", "hi", 1))         # hi world hello，最多替换 1 次

# 拆分
"a,b,c".split(",")             # ['a', 'b', 'c']
"a b  c".split()               # ['a', 'b', 'c']  不带参数按连续空白拆
"a,b,c".rsplit(",", 1)         # ['a,b', 'c']  从右拆一次
"line1\nline2\nline3".splitlines()  # ['line1', 'line2', 'line3']

# 拼接：str.join(iterable) 是标准做法
",".join(["a", "b", "c"])      # 'a,b,c'
"\n".join(["行1", "行2"])       # '行1\n行2'
"".join(["a", "b", "c"])       # 'abc'

# ❌ 慢，代码丑
result = ""
for word in words:
    result += word + " "

# ✅ 快，代码美
result = " ".join(words)

"5".zfill(4)                # '0005'  左侧补 0

"abc".ljust(10, "-")        # 'abc-------'
"abc".rjust(10, "-")        # '-------abc'
"abc".center(10, "-")       # '---abc----'

s = "咖飞"
b = s.encode("utf-8")       # b'\xe5\x92\x96\xe9\xa3\x9e'   bytes 对象
s2 = b.decode("utf-8")      # '咖飞'

name = "咖飞"
age = 3

# 1. f-string（Python 3.6+，最推荐）
print(f"我叫 {name}，{age} 岁")
print(f"两位小数：{3.14159:.2f}")     # 3.14
print(f"补 4 位：{7:04d}")             # 0007

# 2. str.format
print("我叫 {}，{} 岁".format(name, age))
print("我叫 {n}，{a} 岁".format(n=name, a=age))

# 3. % 老式，兼容 C printf
print("我叫 %s，%d 岁" % (name, age))

import string

print(string.ascii_letters)     # abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
print(string.ascii_lowercase)   # abcdefghijklmnopqrstuvwxyz
print(string.digits)            # 0123456789
print(string.punctuation)       # !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~
print(string.whitespace)        # 空白字符（含 \n \t）

import random
import string

def gen_password(length: int = 12) -> str:
    chars = string.ascii_letters + string.digits + string.punctuation
    return "".join(random.choices(chars, k=length))

print(gen_password())

s = "咖飞"
lst = list(s)                # ['咖', '飞']
back = "".join(lst)          # '咖飞'

# 单词切分
sentence = "I love Python"
words = sentence.split()     # ['I', 'love', 'Python']
back = " ".join(words)       # 'I love Python'

# mask.py
"""把日志里的手机号和邮箱替换成脱敏形式。"""

def mask_phone(phone: str) -> str:
    """138****1234 形式。"""
    if len(phone) != 11 or not phone.isdigit():
        return phone
    return phone[:3] + "*" * 4 + phone[-4:]


def mask_email(email: str) -> str:
    """把 @ 前只保留首尾两个字符。"""
    if "@" not in email:
        return email
    name, domain = email.split("@", 1)
    if len(name) <= 2:
        masked = name
    else:
        masked = name[0] + "*" * (len(name) - 2) + name[-1]
    return f"{masked}@{domain}"


def mask_line(line: str) -> str:
    """对一行日志同时脱敏（简化版：这里只演示 str 方法，实际生产建议用 re）。"""
    words = line.split()
    return " ".join(
        mask_email(w) if "@" in w else
        mask_phone(w) if len(w) == 11 and w.isdigit() else
        w
        for w in words
    )


if __name__ == "__main__":
    logs = [
        "user 13812345678 login ok",
        "notify kafei@example.com 已发送",
        "顺序无关 admin 18899990000 alice@a.cn",
    ]
    for line in logs:
        print(mask_line(line))

"  a  b  ".split()       # ['a', 'b']
"  a  b  ".split(" ")    # ['', '', 'a', '', 'b', '', '']

print(7 + 3)      # 10   加
print(7 - 3)      # 4    减
print(7 * 3)      # 21   乘
print(7 / 3)      # 2.3333333333333335   除（结果为 float）
print(7 // 3)     # 2    整除（向下取整）
print(7 % 3)      # 1    取余
print(7 ** 3)     # 343  幂运算 7^3
print(-7 // 3)    # -3！负数整除向下取整，不是向零截断
print(-7 % 3)     # 2    结果与除数同号

print("ab" + "cd")     # abcd  拼接
print("-" * 10)        # ----------
print([1, 2] + [3])    # [1, 2, 3]
print([0] * 5)         # [0, 0, 0, 0, 0]

print(3 < 5)       # True
print(3 <= 3)      # True
print(3 > 5)       # False
print(3 == 3)      # True
print(3 != 5)      # True

age = 25
if 18 <= age < 60:
    print("成年人")

# 等价于但不推荐
if age >= 18 and age < 60:
    print("成年人")

x = 10
x += 5    # x = x + 5   → 15
x -= 3    # x = x - 3   → 12
x *= 2    # x = x * 2   → 24
x //= 5   # x = x // 5  → 4
x **= 2   # x = x ** 2  → 16

# 传统写法
line = input()
while line != "quit":
    print(f"你说：{line}")
    line = input()

# 用海象运算符
while (line := input()) != "quit":
    print(f"你说：{line}")

# 传统写法
data = f.read(1024)
while data:
    process(data)
    data = f.read(1024)

# 海象写法
while data := f.read(1024):
    process(data)

age = 20
has_ticket = True

if age >= 18 and has_ticket:
    print("可以进场")

if age < 12 or age > 65:
    print("儿童或老人票")

if not has_ticket:
    print("没票别想混进来")

# 提供默认值
name = user_input or "匿名用户"

# 防御性访问
result = data and data.get("value")   # data 是 None 就不 .get()

print(3 and 5)      # 5
print(0 and 5)      # 0
print(3 or 5)       # 3
print(0 or 5)       # 5
print(None or "默认")  # "默认"

a = 0b1100    # 12
b = 0b1010    # 10

print(bin(a & b))   # 0b1000  按位与
print(bin(a | b))   # 0b1110  按位或
print(bin(a ^ b))   # 0b0110  按位异或
print(bin(~a))      # -0b1101 按位取反
print(bin(a << 2))  # 0b110000 左移 2 位（相当于 *4）
print(bin(a >> 1))  # 0b110    右移 1 位（相当于 //2）