简体繁体 English

了解python反向切片（[::-1]）

[英]Understanding pythons reverse slice ( [::-1] )

原文 2016-03-26 17:41:23 0 1 python/ slice

I always thought that omitting arguments in the python slice operation would result into: 我一直认为忽略python slice操作中的参数会导致：

start = 0 开始= 0
end = len(lst) 结束= len(lst)
step = 1 步= 1

That holds true if the step is positive, but as soon as the step is negative, like in the "reverse slice" [::-1] , omitting start/end results in: 如果步长为正，则成立，但步长为负时，如“ reverse slice” [::-1] ，省略开始/结束将导致：

start = len(lst)-1 开始= len(lst)-1
end = None 结束= None

Is this a special case, or am I missing something? 这是特例，还是我错过了什么？

1 个解决方案

The default is always None ; 默认值始终为 None ; it is up to the type to determine how to handle None for any of the 3 values. 由类型决定如何None三个值中的任何一个处理None 。 The list object is simply passed a slice(None, None, -1) object in this case. 在这种情况下slice(None, None, -1)只将list对象传递给slice(None, None, -1)对象。

See footnote 5 to the operations table in the sequence types documentation for how Python's default sequence types (including list objects) interpret these: 有关Python的默认序列类型（包括列表对象）如何解释这些信息，请参见序列类型文档中操作表的脚注5：

s[i:j:k]
5. [...] If i or j are omitted or None , they become “end” values (which end depends on the sign of k ). 5. [...]如果省略i或j或None ，它们将成为“结束”值（该结束取决于k的符号）。

So the defaults are dependent on the sign of the step value; 因此，默认值取决于步长值的符号。 if negative the ends are reversed. 如果为负，则两端颠倒。 For [::-1] the end values are len(s) - 1 and -1 (absolute, not relative to the end), respectively, because the step is negative. 对于[::-1] ，最终值分别为len(s) - 1和-1 （绝对值，不是相对于最终值），因为步长为负数。