Slicing lists and strings to negative zero

Question

I've read the Python informal tutorial on slicing, which all makes sense to me except for one edge case. It seems like

'help'[:-0]

should evaluate to 'help', but it really evaluates to ''. What's a good way to think about negative index slicing so that this particular edge case makes sense?

Håvard · Accepted Answer · 2011-04-01T23:53:51.733

8

'help'[:-0] is actually equal to 'help'[:0], in which case it makes sense that it evaluates to ''. In fact, as you can see from the interactive python interpreter, -0 is the same as 0:

>>> -0
0

edited Apr 01 '11 at 23:53

answered Apr 01 '11 at 23:30

Håvard

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That's not an "assume". You assumed very little. You actually ran actual code and actually proved what the actual result is. Drop the "assume" business from this otherwise good answer. – S.Lott Apr 01 '11 at 23:51

score 3 · Answer 2 · edited May 23 '17 at 12:04

3

-0 == 0, so 'help'[:-0] is equivalent to 'help'[0:0], which I think you will agree should be ''.

The following question has some good general info on slices and how to think of them: Explain Python's slice notation

edited May 23 '17 at 12:04

Community

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answered Apr 01 '11 at 23:31

Andrew Clark

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fede_lcc · Answer 3 · 2021-02-25T09:27:50.480

2

As others have said, -0 == 0 therefore the output '' makes complete sense.

In case you want a consistent behaviour when using negative indices, you could use len() in the slicing index:

>>> a = 'help'
>>> a[:len(a)-0]
'help'

edited Feb 25 '21 at 09:27

answered Nov 10 '20 at 15:29

fede_lcc

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Indeed, `-0 == 0`. Welcome to SO ! – WayToDoor Nov 10 '20 at 15:39

score 1 · Answer 4 · answered Apr 01 '11 at 23:36

As the others have said, -0 == 0, in which case the '' result is correct. I think you're looking for:

'help'[:]

When slicing, if you omit the start it starts from 0, if you omit the end, it advances until the end of the collection (in your case a string). Thus, [:] means "beginning to end".

Slicing lists and strings to negative zero

4 Answers4