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reversed()函数是返回序列seq的反向访问的迭代子。参数可以是列表,元组,字符串,不改变原对象。
1》参数是列表
>>> l=[1,2,3,4,5] >>> ll=reversed(l) >>> l [1, 2, 3, 4, 5] >>> ll <listreverseiterator object at 0x06A9E930> >>> for i in ll:#第一次遍历 ... print i, ... 5 4 3 2 1 >>> for i in ll:第二次遍历为空,原因见本文最后 ... print i ... 2》参数是列表
>>> l=[3,4,5,6] >>> ll=reversed(l) >>> l [3, 4, 5, 6] >>> ll <listreverseiterator object at 0x06A07E10> >>> list(ll)#第一次 [6, 5, 4, 3] >>> list(ll)#第二次为空,原因见本文最后 []
3》参数是元组
>>> t=(4,5,6) >>> tt=reversed(t) >>> t (4, 5, 6) >>> tt <reversed object at 0x06A07E50> >>> tuple(tt)#第一次 (6, 5, 4) >>> tuple(tt)#第二次为空,原因见本文最后 ()
4》参数是字符串
>>> s='cba' >>> ss=reversed(s) >>> s 'cba' >>> ss <reversed object at 0x06A07E70> >>> list(ss)#第一次 ['a', 'b', 'c'] >>> list(ss)#第二次为空,原因见本文最后 []
5》参数是字符串
>>> s='1234' >>> ss=reversed(s) >>> s '1234' >>> ss <reversed object at 0x06A94490> >>> ''.join(ss)#第一次 '4321' >>> ''.join(ss)#第二次为空,原因见本文最后 ''
为什么reversed()之后,第二次for循环或第二次list()或第二次tuple()或第二次join()得到的结果为空?我们以第2个例子具体说明一下:
That’s because reversed creates an iterator, which is already spent when you’re calling list(ll) for the second time.
The reason is that ll is not the reversed list itself, but a listreverseiterator. So when you call list(ll) the first time, it iterates over ll and creates a new list from the items output from that iterator.When you do it a second time, ll is still the original iterator and has already gone through all the items, so it doesn’t iterate over anything, resulting in an empty list.
总结:reversed()之后,只在第一次遍历时返回值。
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