squares = []
for x in range(10):
    squares.append(x**2)

squares = [x**2 for x in range(10)]

items = ["a", "b", "c"]
for i, item in enumerate(items):
    print(f"{i}: {item}")

a = {"x": 1, "y": 2}
b = {"y": 3, "z": 4}
merged = a | b  # Python 3.9+
# {'x': 1, 'y': 3, 'z': 4}

name = "世界"
price = 49.9
print(f"你好，{name}！价格是 ¥{price:.2f}")

# 交换变量
a, b = b, a

# 多值解包
first, *middle, last = [1, 2, 3, 4, 5]
# first=1, middle=[2,3,4], last=5

from pathlib import Path

# 比 os.path 更直观
data_dir = Path.home() / "data"
data_dir.mkdir(exist_ok=True)

from collections import defaultdict

counts = defaultdict(int)
for word in ["a", "b", "a"]:
    counts[word] += 1

from itertools import combinations

for a, b in combinations([1, 2, 3], 2):
    print(a, b)
# (1,2) (1,3) (2,3)

import time

def timer(func):
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        print(f"{func.__name__} 耗时 {time.time() - start:.3f}s")
        return result
    return wrapper

from dataclasses import dataclass

@dataclass
class User:
    name: str
    age: int

u = User("张三", 25)
print(u)  # User(name='张三', age=25)