1.4 查找集合中最大最小N个元素

heapq 模块：nlargest()和nsmallest()可以完美解决这个问题。

import heapq
nums = [1, 2, 3, 5, -4]
print(heapq.nlargest(3, nums))  # [5, 3, 2]
print(heapq.nsmallest(3, nums))  # [-4, 1, 2]

两函数能接受一个关键字参数，用于更复杂的数据结构中：

import heapq
stocks = [
    {"name": "IBM", "shares": 100, "price": 91.1},
    {"name": "AAPL", "shares": 50, "price": 543.2},
    {"name": "FB", "shares": 200, "price": 21.09},
    {"name": "HPQ", "shares": 35, "price": 31.75},
    {"name": "YHOO", "shares": 45, "price": 16.35}
]
cheaps = heapq.nsmallest(3, stocks, key=lambda s: s["price"])  # 会以prcie的值进行比较
expensives = heapq.nlargest(3, stocks, key=lambda s: s["price"])