列表推导式进阶

1. 推导式

# 列表推导
squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]

# 字典推导
word_len = {w: len(w) for w in ['hello', 'world']}

# 集合推导
unique = {x % 5 for x in range(20)}

# 嵌套推导
flat = [x for row in matrix for x in row]

2. 生成器表达式

# 惰性求值，节省内存
total = sum(x**2 for x in range(1000000))