英文:
Get indexes chosen when slicing a list
问题
我有一个列表：
things = ["a", "b", "c", "d", "e", "f"]
我还有一个为我提供的切片对象，用于上面的列表。以下是一个示例：
s = slice(-2, None, None)print(things[-2:]) # ['e', 'f']
我的目标是提取切片影响的具体索引。在这个示例中，那将是：
[4, 5]
有人知道我应该如何更加Pythonic地做到这一点吗？我目前的解决方案是：
print([i for i in range(len(things))]展开收缩) # [4, 5]
英文:
I have a list:
things = ["a", "b", "c", "d", "e", "f"]
I also have a slice object provided to me, intended for the list above. Here's an example:
s = slice(-2, None, None)print(things[-2:]) # ['e', 'f']
My goal is to extract the specific indexes the slice impacts. In this example, that would be:
[4, 5]
Does anyone know how I would go about doing this more pythonically? My current solution is:
print([i for i in range(len(things))]展开收缩) # [4, 5]
答案1
得分: 2
这正是slice类的indices方法的目的。
正如内置帮助中所解释的那样：
Help on built-in function indices:indices(...) method of builtins.slice instanceS.indices(len) -> (start, stop, stride)假设有一个长度为len的序列，计算由S描述的扩展切片的起始和停止索引以及步长长度。超出边界的索引将被剪切，与普通切片的处理方式一致。
因此，传递给它将被切片的序列的长度，以获取相应的起始/停止/步长值：
>>> s = slice(-2, None, None)>>>>>> things = ['a', 'b', 'c', 'd', 'e', 'f']>>> things展开收缩['e', 'f']>>> s.indices(len(things))(4, 6, 1)>>> x, y, z = s.indices(len(things))>>> things[x:y:z]['e', 'f']
这当然可以用来构造相应的值范围：
>>> list(range(*s.indices(len(things))))[4, 5]
另外，range对象可以直接进行索引和切片，将它们转换为列表也很简单，只需将它们直接传递给list类型即可。因此：
>>> list(range(len(things))展开收缩)[4, 5]>>> list(range(len(things)))展开收缩[4, 5]
这对于获取实际索引列表更加简单；在其他上下文中（例如，为自定义类型实现__getitem__时），起始/停止/步长值可能更直接有用。
英文:
This is exactly the purpose of the indices method of the slice class.
As explained in the built-in help:
Help on built-in function indices:indices(...) method of builtins.slice instanceS.indices(len) -> (start, stop, stride)Assuming a sequence of length len, calculate the start and stopindices, and the stride length of the extended slice described byS. Out of bounds indices are clipped in a manner consistent with thehandling of normal slices.
Thus, pass it the length of the sequence that will be sliced, in order to get the corresponding start/stop/stride values:
>>> s = slice(-2, None, None)>>>>>> things = ['a', 'b', 'c', 'd', 'e', 'f']>>> things展开收缩['e', 'f']>>> s.indices(len(things))(4, 6, 1)>>> x, y, z = s.indices(len(things))>>> things[x:y:z]['e', 'f']
Which can of course be used to construct the corresponding range of values:
>>> list(range(*s.indices(len(things))))[4, 5]
As an aside, range objects can be indexed and sliced directly, and converting them to lists is as easy as passing them directly to the list type. Thus:
>>> list(range(len(things))展开收缩)[4, 5]>>> list(range(len(things)))展开收缩[4, 5]
This is simpler for getting an actual list of indices; in other contexts (for example, implementing __getitem__ for a custom type) the start/stop/stride values may be more directly useful.
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