Python3 sorted() 函数

定义：

def sorted(*args, **kwargs): # real signature unknown
    """
    Return a new list containing all items from the iterable in ascending order.
    
    A custom key function can be supplied to customize the sort order, and the
    reverse flag can be set to request the result in descending order.
    """
    pass

给它一个可迭代对象，返回一个按照升序的新的列表

用户可以自定义关键函数来实现排序，也可以设置反向标志设置为降序。

用法：

sorted(iterable, key=None, reverse=False)

key可以为自定义函数，函数只有一个参数，具体的函数的参数就是取自于可迭代对象中，指定可迭代对象中的一个元素来进行排序。

reverse 默认False，代表升序，设置为True的时候，代表降序。

代码举例一：

import collections
from random import choice

Card = collections.namedtuple('Card', ['rank', 'suit'])


class FrenchDeck:
    ranks = [str(n) for n in range(3, 11)] + list("JQKA2")
    suits = 'spades dimonds clubs hearts'.split(" ")

    def __init__(self):
        self._cards = [Card(rank, suit) for rank in self.ranks
                       for suit in self.suits]

    def __len__(self):
        return len(self._cards)

    def __getitem__(self, position):
        return self._cards[position]


deck = FrenchDeck()
suit_values = dict(spades=3, dimonds=2, clubs=1, hearts=0)


def spades_high(card):
    rank_value = FrenchDeck.ranks.index(card.rank)
    return rank_value * 4 + suit_values[card.suit]


for card in sorted(deck, key=spades_high):
    print(card)

代码中，deck是个可迭代对象，deck打印出如下

Card(rank='3', suit='spades')
Card(rank='3', suit='dimonds')
Card(rank='3', suit='clubs')
Card(rank='3', suit='hearts')
Card(rank='4', suit='spades')
Card(rank='4', suit='dimonds')
Card(rank='4', suit='clubs')
Card(rank='4', suit='hearts')
Card(rank='5', suit='spades')
Card(rank='5', suit='dimonds')
Card(rank='5', suit='clubs')
Card(rank='5', suit='hearts')
Card(rank='6', suit='spades')
Card(rank='6', suit='dimonds')
Card(rank='6', suit='clubs')
Card(rank='6', suit='hearts')
Card(rank='7', suit='spades')
Card(rank='7', suit='dimonds')
Card(rank='7', suit='clubs')
Card(rank='7', suit='hearts')
Card(rank='8', suit='spades')
Card(rank='8', suit='dimonds')
Card(rank='8', suit='clubs')
Card(rank='8', suit='hearts')
Card(rank='9', suit='spades')
Card(rank='9', suit='dimonds')
Card(rank='9', suit='clubs')
Card(rank='9', suit='hearts')
Card(rank='10', suit='spades')
Card(rank='10', suit='dimonds')
Card(rank='10', suit='clubs')
Card(rank='10', suit='hearts')
Card(rank='J', suit='spades')
Card(rank='J', suit='dimonds')
Card(rank='J', suit='clubs')
Card(rank='J', suit='hearts')
Card(rank='Q', suit='spades')
Card(rank='Q', suit='dimonds')
Card(rank='Q', suit='clubs')
Card(rank='Q', suit='hearts')
Card(rank='K', suit='spades')
Card(rank='K', suit='dimonds')
Card(rank='K', suit='clubs')
Card(rank='K', suit='hearts')
Card(rank='A', suit='spades')
Card(rank='A', suit='dimonds')
Card(rank='A', suit='clubs')
Card(rank='A', suit='hearts')
Card(rank='2', suit='spades')
Card(rank='2', suit='dimonds')
Card(rank='2', suit='clubs')
Card(rank='2', suit='hearts')

spades_high是自定义函数，此函数参数取自于可迭代对象deck中，此处为deck中52个由namedtuple创建的名叫Card的元祖对象，通过对spades_high函数的输出数值进行升序排序。

最后排序之后的输出如下：

Card(rank='3', suit='hearts')
Card(rank='3', suit='clubs')
Card(rank='3', suit='dimonds')
Card(rank='3', suit='spades')
Card(rank='4', suit='hearts')
Card(rank='4', suit='clubs')
Card(rank='4', suit='dimonds')
Card(rank='4', suit='spades')
Card(rank='5', suit='hearts')
Card(rank='5', suit='clubs')
Card(rank='5', suit='dimonds')
Card(rank='5', suit='spades')
Card(rank='6', suit='hearts')
Card(rank='6', suit='clubs')
Card(rank='6', suit='dimonds')
Card(rank='6', suit='spades')
Card(rank='7', suit='hearts')
Card(rank='7', suit='clubs')
Card(rank='7', suit='dimonds')
Card(rank='7', suit='spades')
Card(rank='8', suit='hearts')
Card(rank='8', suit='clubs')
Card(rank='8', suit='dimonds')
Card(rank='8', suit='spades')
Card(rank='9', suit='hearts')
Card(rank='9', suit='clubs')
Card(rank='9', suit='dimonds')
Card(rank='9', suit='spades')
Card(rank='10', suit='hearts')
Card(rank='10', suit='clubs')
Card(rank='10', suit='dimonds')
Card(rank='10', suit='spades')
Card(rank='J', suit='hearts')
Card(rank='J', suit='clubs')
Card(rank='J', suit='dimonds')
Card(rank='J', suit='spades')
Card(rank='Q', suit='hearts')
Card(rank='Q', suit='clubs')
Card(rank='Q', suit='dimonds')
Card(rank='Q', suit='spades')
Card(rank='K', suit='hearts')
Card(rank='K', suit='clubs')
Card(rank='K', suit='dimonds')
Card(rank='K', suit='spades')
Card(rank='A', suit='hearts')
Card(rank='A', suit='clubs')
Card(rank='A', suit='dimonds')
Card(rank='A', suit='spades')
Card(rank='2', suit='hearts')
Card(rank='2', suit='clubs')
Card(rank='2', suit='dimonds')
Card(rank='2', suit='spades')

代码举例二：

a = [-5, -3, -1, 0, 2, 4, 6]
print(sorted(a, key=lambda x: x**2))
print(sorted(a, key=lambda x: x**2, reverse=True))

[0, -1, 2, -3, 4, -5, 6]
[6, -5, 4, -3, 2, -1, 0]

即按照lambda函数：x的平方值的升序，来对列表a排序，通过设置reverse标志，来设置升序或者降序。