Python內建函數(54)——reversed

標籤：tor   cal   error:   log   color   迭代器   cti   lin   for   

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).

說明：

　　1. 函數功能是反轉一個序列對象，將其元素從後向前顛倒構建成一個新的迭代器。

>>> a = reversed(range(10)) # 傳入range對象>>> 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. 如果參數不是一個序列對象，則其必須定義一個__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) # 執行個體不能反轉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) # 清單類型<class ‘list‘># 重新定義類型，並為其定義__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 # 清單類型[78, 85, 93, 96]>>> type(a.scores)<class ‘list‘>>>> reversed(a)  # 執行個體變得可以反轉>>> a.scores # 反轉後類型變成迭代器<list_reverseiterator object at 0x0342F3B0>>>> type(a.scores)<class ‘list_reverseiterator‘>>>> list(a.scores)[96, 93, 85, 78]

 

Python內建函數(54)——reversed