英文文檔：

reversed(seq)

Return a reverse iterator. seq must be an object which has a __reversed__() method or supports the sequence protocol (the __len__() method and the __getitem__() method with integer arguments starting at 0).

反轉(zhuǎn)序列生成新的可迭代對(duì)象

說明：

1. 函數(shù)功能是反轉(zhuǎn)一個(gè)序列對(duì)象，將其元素從后向前顛倒構(gòu)建成一個(gè)新的迭代器。

>>> a = reversed(range(10)) # 傳入range對(duì)象>>> a # 類型變成迭代器<range_iterator object at 0x035634E8>>>> list(a)[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]>>> a = [’a’,’b’,’c’,’d’]>>> a[’a’, ’b’, ’c’, ’d’]>>> reversed(a) # 傳入列表對(duì)象<list_reverseiterator object at 0x031874D0>>>> b = reversed(a)>>> b # 類型變成迭代器<list_reverseiterator object at 0x037C4EB0>>>> list(b)[’d’, ’c’, ’b’, ’a’]

2. 如果參數(shù)不是一個(gè)序列對(duì)象，則其必須定義一個(gè)__reversed__方法。

# 類型Student沒有定義__reversed__方法>>> class Student: def __init__(self,name,*args): self.name = name self.scores = [] for value in args: self.scores.append(value) >>> a = Student(’Bob’,78,85,93,96)>>> reversed(a) # 實(shí)例不能反轉(zhuǎn)Traceback (most recent call last): File '<pyshell#37>', line 1, in <module> reversed(a)TypeError: argument to reversed() must be a sequence>>> type(a.scores) # 列表類型<class ’list’># 重新定義類型，并為其定義__reversed__方法>>> class Student: def __init__(self,name,*args): self.name = name self.scores = [] for value in args: self.scores.append(value) def __reversed__(self): self.scores = reversed(self.scores) >>> a = Student(’Bob’,78,85,93,96)>>> a.scores # 列表類型[78, 85, 93, 96]>>> type(a.scores)<class ’list’>>>> reversed(a) # 實(shí)例變得可以反轉(zhuǎn)>>> a.scores # 反轉(zhuǎn)后類型變成迭代器<list_reverseiterator object at 0x0342F3B0>>>> type(a.scores)<class ’list_reverseiterator’>>>> list(a.scores)[96, 93, 85, 78]

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