Creating a slice object in python

Question

If I have an array a, I understand how to slice it in various ways. Specifically, to slice from an arbitrary first index to the end of the array I would do a[2:].

But how would I create a slice object to achieve the same thing? The two ways to create slice objects that are documented are slice(start, stop, step) and slice(stop).

So if I pass a single argument like I would in a[2:] the slice object would interpret it as the stopping index rather than the starting index.

Question: How do I pass an index to the slice object with a starting index and get a slice object that slices all the way to the end? I don't know the total size of the list.

What are you trying to slice? Are you looking for something like stackoverflow.com/questions/2936863/… ? — Jasper
– Jasper, Commented Jun 4, 2016 at 13:24
Use "None" for the blank sections. So the reversing idiom [::-1] could be created with: reversing_slice=slice(None,None,-1) — RufusVS
– RufusVS, Commented Oct 5, 2018 at 15:24
@RufusVS: Why add a comment that replicates the accepted answer two years after it was posted? — ShadowRanger
– ShadowRanger, Commented Sep 5, 2019 at 16:01
@ShadowRanger I must have commented before reading the answers, because I see I upvoted up the accepted answer as well. I'm older and (hopefully) wiser now. — RufusVS
– RufusVS, Commented Sep 5, 2019 at 21:57

ShadowRanger · Accepted Answer · 2016-06-04 13:28:20Z

19

Use None everywhere the syntax-based slice uses a blank value:

someseq[slice(2, None)]

is equivalent to:

someseq[2:]

Similarly, someseq[:10:2] can use a preconstructed slice defined with slice(None, 10, 2), etc.

answered Jun 4, 2016 at 13:28

ShadowRanger

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