Python內置函數(54)——reversed

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，將其元素從后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) # 實例不能反轉
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) # 列表類型
<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 # 列表類型
[78, 85, 93, 96]
>>> type(a.scores)
<class 'list'>

>>> reversed(a)  # 實例變得可以反轉
>>> a.scores # 反轉后類型變成迭代器
<list_reverseiterator object at 0x0342F3B0>
>>> type(a.scores)
<class 'list_reverseiterator'>

>>> list(a.scores)
[96, 93, 85, 78]