第 3 章 · 进阶特性

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(nums[0:3])     # 前 3 个 → [0,1,2]
print(nums[-3:])     # 最后 3 个 → [7,8,9]
print(nums[::2])     # 隔一个取 → [0,2,4,6,8]
print(nums[::-1])    # 反转 → [9,8,...,0]

text = "上海自来水来自海上"
print(text == text[::-1])   # 回文？把句子倒过来和原句比

prices = ["￥12", "￥8", "￥20"]
print(prices[1:])           # 从第 2 个开始取一段

path = "data/2024_report_final.csv"
name = path.split("/")[-1]
ext = name[-3:]             # 实战：取文件扩展名（后 3 个字符）
print("文件名：", name)
print("扩展名：", ext)

# 凡是能用 for 挨个取出来的，都叫“可迭代”
for ch in "知码":        # 字符串可迭代
    print(ch)

# dict 默认迭代“键”，想要“键值对”用 .items()
d = {"a": 1, "b": 2}
for k, v in d.items():
    print(k, "→", v)

class BookShelf:
    def __init__(self, books):
        self.books = books
        self.i = 0
    def __iter__(self):
        return self
    def __next__(self):
        if self.i >= len(self.books):
            raise StopIteration
        book = self.books[self.i]
        self.i += 1
        return book

shelf = BookShelf(["三体", "活着", "围城"])
for b in shelf:
    print("读：", b)

scores = [88, 92, 75]
for idx, s in enumerate(scores, 1):   # 实战：顺便拿到名次
    print(f"第{idx}名：{s}分")

squares = [x * x for x in range(1, 6)]     # 1~5 的平方
print(squares)

# 带条件的生成式：只要偶数
evens = [x for x in range(10) if x % 2 == 0]
print(evens)

# 把字符串列表全变成大写
words = ["python", "data", "ai"]
upper = [w.upper() for w in words]
print(upper)

celsius = [0, 20, 37]
fahrenheit = [round(c * 9 / 5 + 32, 1) for c in celsius]   # 生活：摄氏转华氏
print(fahrenheit)

logs = ["OK 200", "ERR 404", "OK 200", "ERR 500"]
err_codes = [line.split()[1] for line in logs if line.startswith("ERR")]   # 实战：从日志揪出错误码
print(err_codes)

def count_up(n):
    i = 0
    while i < n:
        yield i        # 每次“暂停”在这里，返回一个值
        i += 1

g = count_up(3)
print(next(g))   # 0
print(next(g))   # 1
print(list(count_up(3)))   # [0, 1, 2]

def fib():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

f = fib()
for _ in range(6):
    print(next(f), end=" ")
print()

import io
fake_file = io.StringIO("hello\n\nworld\n\npython\n")   # 模拟一个大文件

def blank_lines(lines):                 # 实战：逐行扫描，遇到空行才“产出”
    for line in lines:
        if line.strip() == "":
            yield "空行"

blanks = list(blank_lines(fake_file))
print("空行数：", len(blanks))

nums = [1, 2, 3, 4, 5]
doubled = list(map(lambda x: x * 2, nums))   # 每个都乘以 2
print(doubled)                               # [2,4,6,8,10]

big = list(filter(lambda x: x > 2, nums))     # 只保留 >2 的
print(big)                                   # [3,4,5]

# 其实用列表生成式更直观，二者等价：
print([x * 2 for x in nums])

prices = ["10.5", "3.0", "20.0", "8.5"]
floats = list(map(float, prices))            # 生活：字符串价格 → 数字
print(floats)
cheap = list(filter(lambda p: p < 10, floats))   # 只要便宜的
print(cheap)

students = [("小明", 88), ("小红", 95), ("小刚", 72)]
ranked = sorted(students, key=lambda s: s[1], reverse=True)   # 实战：按成绩排名
print(ranked)

from functools import wraps

def log_calls(func):              # 装饰器：接收原函数
    @wraps(func)                  # 保住原函数的名字、文档
    def wrapper(*args, **kw):
        print(f"调用 {func.__name__}，参数 {args} {kw}")
        return func(*args, **kw)
    return wrapper

@log_calls                     # 用 @ 把装饰器“贴”在函数上
def add(a, b):
    return a + b

print(add(3, 4))

def retry(times):
    def deco(func):
        def wrapper(*args, **kw):
            for i in range(times):
                try:
                    return func(*args, **kw)
                except Exception:
                    print(f"第{i+1}次失败，重试…")
            return None
        return wrapper
    return deco

@retry(2)
def unstable():
    raise ValueError("网络抖了一下")

print(unstable())

cache = {}
def memo(func):
    def wrapper(n):
        if n in cache:
            return cache[n]
        cache[n] = func(n)
        return cache[n]
    return wrapper

@memo
def fib(n):
    if n < 2:
        return n
    return fib(n-1) + fib(n-2)

print(fib(10))   # 第 11 个斐波那契数

import functools
def tag(func):
    @functools.wraps(func)        # 保住原函数元信息
    def wrapper():
        return func()
    return wrapper

@tag
def hello():
    return "hi"

print(hello.__name__)   # hello（不再是 wrapper）