为什么我不能将任意迭代分配给步长为 -1 的扩展切片? [英] Why can't I assign an arbitrary iterable to an extended slice whose step is -1?

问题描述

Python 3.2.3 (default, Apr 11 2012, 07:15:24) [MSC v.1500 32 bit (Intel)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>> u = [4, 5, 6, 7, 8, 9]
>>> u[1::1] = [3, 2, 1, 0]
>>> u
[4, 3, 2, 1, 0]
>>> u[9:0:-1] = [8, 7, 6, 5]
>>> u
[4, 5, 6, 7, 8]
>>> u[9:0:-1] = [16, 12, 8]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: attempt to assign sequence of size 3 to extended slice of size 4
>>> u
[4, 5, 6, 7, 8]
>>>

预期行为:在最终赋值语句中没有抛出异常；u 应该在最后一行打印为 [4, 8, 12, 16].

Expected behaviour: no exception thrown on final assignment statement; u should print on final line as [4, 8, 12, 16].

我可以分配给步长为 1 的扩展切片，即使我分配的可迭代对象是错误的长度".为什么我不能分配给步长为 -1 的扩展切片并让它以明显的方式工作?

I can assign to an extended slice whose step is 1, even if the iterable I'm assigning is "the wrong length". Why then can't I assign to an extended slice whose step is -1 and have it work in the obvious way?

推荐答案

我认为创建一个步长为 1 的扩展切片有效地像一个常规切片而不是扩展切片.

I think that creating an extended slice whose step is 1 effectively acts like a regular slice rather than an extended slice.

扩展切片不允许您更改序列的长度，如这里

Extended slices do not allow you to change the length of the sequence, as noted here

如果您有一个可变序列，例如列表或数组，您可以分配或删除扩展切片，但分配给扩展切片和常规切片之间存在一些差异.对常规切片的赋值可用于更改序列的长度.扩展切片不是那么灵活.分配给扩展切片时，语句右侧的列表必须包含与其要替换的切片相同数量的项目.

If you have a mutable sequence such as a list or an array you can assign to or delete an extended slice, but there are some differences between assignment to extended and regular slices. Assignment to a regular slice can be used to change the length of the sequence. Extended slices aren't this flexible. When assigning to an extended slice, the list on the right hand side of the statement must contain the same number of items as the slice it is replacing.

至于为什么会这样，我只能猜测是因为没有明显行为的情况.举个例子:

As for why it's works this way, I can only guess it is because of the cases where there is no obvious behaviour. Take this example:

u = [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
u[0:8:3] = [ 10, 11 ]

您希望它如何工作?我想你可以只替换 1 &4 与 10 &11，但是7呢?你留下吗?删除它?删除 7 之后的整个序列?也许只有我一个人，但这个案子似乎不太清楚.我认为这就是为什么扩展切片不允许这种行为的原因.

How would you expect this to work? I guess you could just replace 1 & 4 with 10 & 11, but what about 7? Do you leave it? Delete it? Delete the entire rest of the sequence past 7? Maybe it's just me, but this case doesn't seem too clear cut. Which I would assume is why this sort of behaviour just wasn't allowed for extended slices.

这篇关于为什么我不能将任意迭代分配给步长为 -1 的扩展切片?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！