print("Hello, World!")
print("你好，Python！")

name = "Python"
age = 34
pi = 3.14159
is_awesome = True

print(name, type(name))
print(age, type(age))
print(pi, type(pi))
print(is_awesome, type(is_awesome))

name = "Python"
print(f"你好，{name}！欢迎学习 Python。")

# f-string 中可以直接写表达式
print(f"明年你就 {18 + 1} 岁了")

# 算术运算
print(7 / 2)    # 3.5  — 普通除法
print(7 // 2)   # 3    — 整除（向下取整）
print(7 % 2)    # 1    — 取余
print(2 ** 10)  # 1024 — 幂运算（不是 ^）
print(-7 // 2)  # -4   — 注意：向下取整

# Python 的 int 没有溢出
print(2 ** 100)  # 精确的大整数

s = "Hello, Python"

# 索引（可为负数）
print(s[0])     # H
print(s[-1])    # n

# 切片 [start:stop:step]
print(s[0:5])   # Hello
print(s[7:])    # Python
print(s[::-1])  # nohtyP, olleH — 反转

# 常用操作
print(len(s))           # 13
print("Python" in s)    # True
print(s.upper())        # HELLO, PYTHON
print(s.replace("Python", "World"))

print(3 > 2 and 2 > 1)   # True
print(3 > 5 or 2 > 1)    # True
print(not True)          # False

# 链式比较 — Python 独有
score = 85
print(60 <= score <= 100) # True

# 成员运算符
print(3 in [1, 2, 3])     # True
print(5 not in [1, 2, 3]) # True

# 身份运算符 is
a = [1, 2]
b = a
c = [1, 2]
print(a is b)  # True — 同一个对象
print(a is c)  # False — 值相同但不是同一对象
print(a == c)  # True — 值相等

# 这些都是 False
print(bool(0))      # False
print(bool(""))     # False
print(bool([]))     # False
print(bool(None))   # False

# 这些都是 True
print(bool(42))     # True
print(bool("hi"))   # True
print(bool([0]))    # True（非空列表）

score = 78

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"成绩: {score}, 等级: {grade}")

age = 20
status = "成年" if age >= 18 else "未成年"
print(status)

# match-case (Python 3.10+)
command = "start"
match command:
    case "start":
        print("启动")
    case "stop":
        print("停止")
    case _:
        print("未知命令")

# 经典：1 到 100 求和
total = 0
i = 1
while i <= 100:
    total += i
    i += 1
print(f"1+2+...+100 = {total}")  # 5050

# break：猜数字
attempts = 0
while True:
    attempts += 1
    if attempts >= 3:
        print(f"超过 {attempts} 次")
        break
    print(f"第 {attempts} 次尝试")

# while-else：循环正常结束才执行
n = 7
d = 2
while d < n:
    if n % d == 0:
        print(f"{n} 不是素数")
        break
    d += 1
else:
    print(f"{n} 是素数")  # 没有被 break，执行 else

# range(start, stop, step) — 左闭右开
for i in range(5):        # 0,1,2,3,4
    print(i, end=" ")
print()

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

# 直接遍历字符串
for ch in "Python":
    print(ch, end=" ")
print()

# enumerate：同时获取索引和值
fruits = ["苹果", "香蕉", "橙子"]
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# zip：并行遍历多个序列
names = ["Alice", "Bob", "Carol"]
scores = [90, 85, 78]
for name, score in zip(names, scores):
    print(f"{name}: {score}分")

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

# 创建列表
nums = [3, 1, 4, 1, 5, 9, 2, 6]
mixed = [1, "hello", True, 3.14]

# 索引与切片（和字符串一样）
print(nums[0])    # 3
print(nums[-1])   # 6
print(nums[2:5])  # [4, 1, 5]
print(nums[::-1]) # 反转

# 修改
nums[0] = 99
nums.append(100)       # 末尾添加
nums.insert(1, 50)     # 指定位置插入
nums.remove(1)         # 删除第一个值为 1 的元素
popped = nums.pop()    # 弹出末尾
print(nums)

# 排序
nums = [3, 1, 4, 1, 5, 9, 2, 6]

# sorted() 返回新列表，不改原列表
print(sorted(nums))           # [1, 1, 2, 3, 4, 5, 6, 9]
print(sorted(nums, reverse=True))  # 降序

# .sort() 原地排序，返回 None
nums.sort()
print(nums)

# 用 key 自定义排序规则
words = ["banana", "apple", "cherry"]
words.sort(key=len)
print(words)  # ['apple', 'banana', 'cherry']

# 传统写法
squares = []
for i in range(10):
    squares.append(i ** 2)

# 列表推导式 — 一行搞定
squares = [i ** 2 for i in range(10)]
print(squares)

# 带条件过滤
evens = [i for i in range(20) if i % 2 == 0]
print(evens)

# 带条件表达式
labels = ["偶" if i % 2 == 0 else "奇" for i in range(5)]
print(labels)

# 嵌套推导式
matrix = [[i * 3 + j for j in range(3)] for i in range(3)]
print(matrix)
# [[0, 1, 2], [3, 4, 5], [6, 7, 8]]

# 字典/集合推导式
sq_dict = {i: i**2 for i in range(5)}
print(sq_dict)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# 创建元组
point = (3, 4)
single = (42,)    # 单元素元组必须加逗号

# 不可变
# point[0] = 5  # TypeError!

# 解包（unpacking）
x, y = point
print(x, y)  # 3 4

# 交换变量 — 不需要临时变量
a, b = 1, 2
a, b = b, a
print(a, b)  # 2 1

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

# 函数返回多个值（本质是返回元组）
def min_max(nums):
    return min(nums), max(nums)

lo, hi = min_max([3, 1, 4, 1, 5, 9])
print(f"最小: {lo}, 最大: {hi}")

# enumerate 返回的也是元组
for index, value in enumerate(["a", "b", "c"]):
    print(f"{index} -> {value}")

# 创建
student = {"name": "张三", "age": 20, "major": "CS"}

# 访问
print(student["name"])         # 张三
print(student.get("gpa", 0))   # 0（键不存在返回默认值）

# 增删改
student["gpa"] = 3.8          # 添加/修改
del student["major"]          # 删除
age = student.pop("age")      # 弹出
print(student)

# 遍历
scores = {"数学": 90, "英语": 85, "物理": 92}
for key, value in scores.items():
    print(f"{key} = {value}")

# 字典推导式
nums = [1, 2, 3, 4, 5]
sq = {n: n**2 for n in nums}
print(sq)  # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# 统计词频
text = "the cat sat on the mat the cat"
words = text.split()
freq = {}
for word in words:
    freq[word] = freq.get(word, 0) + 1
print(freq)
# {'the': 3, 'cat': 2, 'sat': 1, 'on': 1, 'mat': 1}

# 更优雅：collections.Counter
from collections import Counter
print(dict(Counter(words)))

# 创建
s1 = {1, 2, 3, 4}
s2 = {3, 4, 5, 6}

# 去重
nums = [1, 2, 2, 3, 3, 3, 4]
unique = list(set(nums))
print(unique)  # [1, 2, 3, 4]

# 集合运算
print(s1 | s2)  # 并集 {1,2,3,4,5,6}
print(s1 & s2)  # 交集 {3,4}
print(s1 - s2)  # 差集 {1,2}
print(s1 ^ s2)  # 对称差 {1,2,5,6}

# 成员判断
print(3 in s1)  # True

# 实用场景：找出两个列表的共同元素
list_a = ["Python", "Java", "C++", "Go"]
list_b = ["Java", "Rust", "Python", "Swift"]

common = set(list_a) & set(list_b)
print(f"共同元素: {common}")

only_a = set(list_a) - set(list_b)
print(f"只在 A 中: {only_a}")

# 基本函数
def greet(name, greeting="你好"):
    return f"{greeting}，{name}！"

print(greet("张三"))          # 你好，张三！
print(greet("李四", "Hi"))    # Hi，李四！
print(greet(name="王五"))     # 关键字参数

# 多返回值
def divide(a, b):
    quotient = a // b
    remainder = a % b
    return quotient, remainder

q, r = divide(17, 5)
print(f"17 / 5 = {q} 余 {r}")

# 可变参数
def sum_all(*args):
    """接收任意数量的位置参数"""
    return sum(args)

print(sum_all(1, 2, 3))       # 6
print(sum_all(1, 2, 3, 4, 5)) # 15

# 关键字可变参数
def make_profile(**kwargs):
    for key, value in kwargs.items():
        print(f"  {key}: {value}")

make_profile(name="Alice", age=25, city="Beijing")

# 混合使用
def func(a, b, *args, **kwargs):
    print(f"a={a}, b={b}, args={args}, kwargs={kwargs}")

func(1, 2, 3, 4, x=5, y=6)

# lambda 基本语法
square = lambda x: x ** 2
print(square(5))  # 25

# lambda 最常用于排序 key
students = [
    ("张三", 85),
    ("李四", 92),
    ("王五", 78),
]

# 按成绩排序
by_score = sorted(students, key=lambda s: s[1], reverse=True)
print(by_score)
# [('李四', 92), ('张三', 85), ('王五', 78)]

# map：对每个元素应用函数
nums = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, nums))
print(squared)  # [1, 4, 9, 16, 25]

# filter：过滤
evens = list(filter(lambda x: x % 2 == 0, nums))
print(evens)  # [2, 4]

# 更 Pythonic 的写法：列表推导式
squared = [x**2 for x in nums]
evens = [x for x in nums if x % 2 == 0]

# reduce：累积运算
from functools import reduce
product = reduce(lambda a, b: a * b, nums)
print(product)  # 120 = 1*2*3*4*5

text = "  Hello, World!  "

# 去空白
print(text.strip())    # "Hello, World!"

# 分割与合并
csv = "张三,25,CS,3.8"
parts = csv.split(",")
print(parts)  # ['张三', '25', 'CS', '3.8']

joined = "-".join(parts)
print(joined)  # 张三-25-CS-3.8

# 查找与替换
s = "Hello, Python!"
print(s.replace("Python", "World"))
print(s.find("Python"))  # 7（索引），找不到返回 -1
print(s.startswith("Hello"))  # True
print(s.count("l"))  # 3

# 格式化字符串
name = "Alice"
score = 95.678

# f-string（推荐）
print(f"姓名: {name}")
print(f"分数: {score:.2f}")  # 保留2位小数: 95.68
print(f"{'居中':^20}")       # 居中填充

# 判断方法
print("123".isdigit())     # True
print("abc".isalpha())     # True
print("Hello123".isalnum()) # True

# 写文件
with open("/tmp/test.txt", "w") as f:
    f.write("第一行\n")
    f.write("第二行\n")
    f.writelines(["第三行\n", "第四行\n"])

# 读文件 — 一次性读完
with open("/tmp/test.txt", "r") as f:
    content = f.read()
    print(content)

# 逐行读（推荐，内存友好）
with open("/tmp/test.txt", "r") as f:
    for line in f:
        print(f"行: {line.strip()}")

# JSON 文件读写
import json

data = {
    "name": "张三",
    "age": 20,
    "courses": ["数学", "英语", "物理"]
}

# 写 JSON
with open("/tmp/data.json", "w") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

# 读 JSON
with open("/tmp/data.json", "r") as f:
    loaded = json.load(f)
    print(loaded)
    print(loaded["courses"])

# 基本结构
try:
    num = int("abc")
    result = 10 / num
except ValueError:
    print("不是有效数字")
except ZeroDivisionError:
    print("不能除以零")
except Exception as e:
    print(f"其他错误: {e}")
else:
    print(f"结果: {result}")  # 没有异常时执行
finally:
    print("无论如何都执行")

# 主动抛出异常
def check_age(age):
    if age < 0:
        raise ValueError("年龄不能为负数")
    return age

try:
    check_age(-5)
except ValueError as e:
    print(f"错误: {e}")

# 自定义异常
class InvalidScoreError(Exception):
    pass

def set_score(score):
    if not 0 <= score <= 100:
        raise InvalidScoreError(f"分数必须在 0-100 之间")
    return score

try:
    set_score(150)
except InvalidScoreError as e:
    print(f"错误: {e}")

class Student:
    # 类变量（所有实例共享）
    school = "清华大学"
    
    # 构造函数
    def __init__(self, name, age, score):
        self.name = name
        self.age = age
        self.score = score
    
    # 实例方法
    def introduce(self):
        return f"我是{self.school}的{self.name}，{self.age}岁"
    
    # 魔法方法
    def __str__(self):
        return f"Student({self.name})"
    
    def __repr__(self):
        return f"Student(name={self.name!r}, age={self.age}, score={self.score})"

s1 = Student("张三", 20, 90)
print(s1.introduce())
print(s1)         # 调用 __str__
print(repr(s1))   # 调用 __repr__

# 类方法与静态方法
class MathUtils:
    pi = 3.14159
    
    @staticmethod
    def is_even(n):
        return n % 2 == 0
    
    @classmethod
    def circle_area(cls, r):
        return cls.pi * r ** 2

print(MathUtils.is_even(4))       # True
print(MathUtils.circle_area(5))  # 78.54...

# 属性装饰器
class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius
    
    @property
    def fahrenheit(self):
        return self._celsius * 9/5 + 32

t = Temperature(100)
print(t.fahrenheit)  # 212.0

class Animal:
    def __init__(self, name):
        self.name = name
    def speak(self):
        return f"{self.name}发出声音"

class Dog(Animal):
    def __init__(self, name, breed):
        super().__init__(name)
        self.breed = breed
    def speak(self):  # 方法重写
        return f"{self.name}（{self.breed}）说：汪汪！"

class Cat(Animal):
    def speak(self):
        return f"{self.name}说：喵喵~"

# 多态
animals = [Dog("旺财", "柴犬"), Cat("咪咪")]
for a in animals:
    print(a.speak())

print(isinstance(Dog("test", "test"), Animal))  # True

# 运算符重载
class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __repr__(self):
        return f"Vector({self.x}, {self.y})"
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y
    def __len__(self):
        return int((self.x**2 + self.y**2) ** 0.5)

v1 = Vector(3, 4)
v2 = Vector(1, 2)
print(v1 + v2)        # Vector(4, 6)
print(v1 * 3)         # Vector(9, 12)
print(v1 == Vector(3, 4))  # True
print(len(v1))        # 5

# 常用标准库
import random
import math
import datetime

# random — 随机数
print(random.randint(1, 100))    # 1-100 随机整数
print(random.choice(["红", "黄", "蓝"]))
print(random.sample(range(1, 50), 6))     # 不重复抽样

# math — 数学函数
print(math.sqrt(144))   # 12.0
print(math.ceil(3.1))   # 4（向上取整）
print(math.floor(3.9))  # 3（向下取整）
print(math.gcd(12, 18)) # 6（最大公约数）

# datetime — 日期时间
now = datetime.datetime.now()
print(now.strftime("%Y-%m-%d %H:%M"))
today = datetime.date.today()
print(today)

# collections — 高级容器
from collections import Counter, defaultdict, deque

# Counter — 计数器
text = "abracadabra"
c = Counter(text)
print(c.most_common(3))  # [('a', 5), ('b', 2), ('r', 2)]

# defaultdict — 带默认值的字典
dd = defaultdict(list)
dd["fruits"].append("apple")
dd["fruits"].append("banana")
print(dd["fruits"])  # ['apple', 'banana']

# itertools — 迭代工具
from itertools import chain, product, combinations
print(list(chain([1, 2], [3, 4])))  # [1, 2, 3, 4]
print(list(combinations("ABC", 2)))  # [('A','B'),...]

# 生成器函数：用 yield 代替 return
def countdown(n):
    while n > 0:
        yield n
        n -= 1

# 生成器是迭代器
for i in countdown(5):
    print(i, end=" ")  # 5 4 3 2 1
print()

# 手动迭代
gen = countdown(3)
print(next(gen))  # 3
print(next(gen))  # 2
print(next(gen))  # 1

# 生成器表达式（类似列表推导式，但用圆括号）
squares = (x**2 for x in range(1000000))
print(next(squares))  # 0 — 不占内存！
print(next(squares))  # 1

# 斐波那契数列生成器
def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

# 取前 10 个
from itertools import islice
fib10 = list(islice(fibonacci(), 10))
print(fib10)  # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

# yield from：委托给子生成器
def flatten(nested):
    for item in nested:
        if isinstance(item, list):
            yield from flatten(item)
        else:
            yield item

print(list(flatten([1, [2, [3, 4]], 5])))
# [1, 2, 3, 4, 5]

import json

class StudentManager:
    def __init__(self):
        self.students = []
    
    def add(self, name, age, score):
        """添加学生"""
        if not 0 <= score <= 100:
            raise ValueError("成绩必须在 0-100 之间")
        self.students.append({
            "name": name, "age": age, "score": score
        })
    
    def list_all(self):
        """列出所有学生"""
        for s in self.students:
            print(f"  {s['name']:<6} {s['age']}岁  {s['score']}分")
    
    def rank(self):
        """按成绩降序"""
        return sorted(self.students, key=lambda s: s["score"], reverse=True)
    
    def stats(self):
        """统计信息"""
        scores = [s["score"] for s in self.students]
        return {
            "人数": len(scores),
            "平均分": round(sum(scores) / len(scores), 1),
            "最高分": max(scores),
            "最低分": min(scores)
        }
    
    def save(self, path):
        with open(path, "w") as f:
            json.dump(self.students, f, ensure_ascii=False, indent=2)

# 使用
mgr = StudentManager()
mgr.add("张三", 20, 90)
mgr.add("李四", 21, 85)
mgr.add("王五", 19, 95)
mgr.add("赵六", 22, 78)

print("=== 所有学生 ===")
mgr.list_all()

print("\n=== 成绩排名 ===")
for i, s in enumerate(mgr.rank(), 1):
    print(f"  第{i}名: {s['name']} {s['score']}分")

print("\n=== 统计 ===")
for k, v in mgr.stats().items():
    print(f"  {k}: {v}")