Python `reversed()`函数

原文： https://thepythonguru.com/python-builtin-functions/reversed/

于 2020 年 1 月 7 日更新

reversed()函数允许我们以相反的顺序处理项目。 它接受一个序列并返回一个迭代器。

其语法如下：

语法：

reversed(sequence) -> reverse iterator

这里有些例子：

>>>
>>> reversed([44, 11, -90, 55, 3])
<list_reverseiterator object at 0x7f2aff2f91d0>
>>>
>>>
>>> list(reversed([44, 11, -90, 55, 3])) # reversing a list
[3, 55, -90, 11, 44]
>>>
>>>
>>> list(reversed((6, 1, 3, 9))) # reversing a tuple
[9, 3, 1, 6]
>>>
>>> list(reversed("hello")) # reversing a string
['o', 'l', 'l', 'e', 'h']
>>>

试试看：

print( reversed([44, 11, -90, 55, 3]) )
print(list(reversed([44, 11, -90, 55, 3]))) # reversing a list
print( list(reversed((6, 1, 3, 9)))) # reversing a tuple
print(list(reversed("hello"))) # reversing a string

为了立即产生结果，我们将reversed()包装在list()调用中。 Python 2 和 Python 3 都需要这样做。

传递给reversed()的参数必须是正确的序列。 尝试传递不保持其顺序（例如dict和set）的对象将导致TypeError。

>>>
>>> reversed({0, 4, -2, 12, 6})
Traceback (most recent call last):
  File "", line 1, in
TypeError: argument to reversed() must be a sequence
>>>
>>>
>>> reversed({'name': 'John', 'age': 20})
Traceback (most recent call last):
  File "", line 1, in
TypeError: argument to reversed() must be a sequence
>>>

反转用户定义的对象

若要反转用户定义的对象，该类必须执行下列操作之一：

1. 实现__len__()和__getitem__()方法； 要么
2. 实现__reversed__()方法

在下面的清单中，CardDeck类实现__len__()和__getitem__()方法。 结果，我们可以将reversed()应用于CardDeck实例。

>>>
>>> from collections import namedtuple
>>>
>>> Card = namedtuple('Card', ['rank', 'suit'])
>>>
>>> class CardDeck:
...     suits = ('club', 'diamond', 'heart', 'spades')
...     ranks = tuple((str(i) for i in range(2, 11))) + tuple("JQKA")
...
...     def __init__(self):
...         self._cards = [Card(r, s) for s in self.suits for r in self.ranks ]
...
...     def __len__(self):
...         return len(self._cards)
...
...     def __getitem__(self, index):
...         return self._cards[index]
...
...    #  def __reversed__(self):   this is how you would define __reversed__() method
...    #     return self._cards[::-1]
...
...
>>>
>>> deck = CardDeck()
>>>
>>> deck
<__main__.CardDeck object at 0x7f2aff2feb00>
>>>
>>>
>>> deck[0], deck[-1] # deck before reversing
(Card(rank='2', suit='club'), Card(rank='A', suit='spades'))
>>>
>>>
>>> reversed_deck = list(reversed(deck))
>>>
>>>
>>> reversed_deck[0], reversed_deck[-1] # deck after reversing
(Card(rank='A', suit='spades'), Card(rank='2', suit='club'))
>>>

试一试：

from collections import namedtuple
Card = namedtuple('Card', ['rank', 'suit'])
class CardDeck:
    suits = ('club', 'diamond', 'heart', 'spades')
    ranks = tuple((str(i) for i in range(2, 11))) + tuple("JQKA")
    def __init__(self):
        self._cards = [Card(r, s) for s in self.suits for r in self.ranks ]
    def __len__(self):
        return len(self._cards)
    def __getitem__(self, index):
        return self._cards[index]
    #  def __reversed__(self):   this is how you would define __reversed__() method
    #     return self._cards[::-1]
deck = CardDeck()
print(deck)
print( deck[0], deck[-1] ) # deck before reversing
reversed_deck = list(reversed(deck))
print(reversed_deck[0], reversed_deck[-1] ) # deck after reversing