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I recently read a code snippets about how to reverse a sequence

>> l = [1,2,3,4,5,6]
>> print l[::-1]

Output

>> [6,5,4,3,2,1]

How to explain the first colon in bracket?

Sukrit Kalra
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lazybios
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  • FYI, that is not the best way to reverse a sequence. Just use `list(reversed(l))`. – anon582847382 Mar 30 '14 at 14:38
  • why don't you get rid of it and see what happens? In other words address the list with only one colon [:-1] – PyNEwbie Mar 30 '14 at 14:38
  • @Alex what? How is that "better"? Maybe it's more general, but if you have a list or string I don't see any advantages to your approach (and it's longer, less idiomatic and probably slower) – Niklas B. Mar 30 '14 at 15:32

2 Answers2

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The colons with no values given means resort to default values. The default values for the start index when the step is negative is len(l), and the end index is -len(l)-1. So, the reverse slicing can be written as 

l[len(l):-len(l)-1:-1]

which is of the form.

l[start:end:step]

Removing the default values, we can use it in a shorter notation as l[::-1].

It might be useful to go through this question on Python's Slice Notation.

Community
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Sukrit Kalra
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    `l[0:len(l):-1]` is something cmpletely different from `l[::-1]`. The equivalent would be `l[len(l):-len(l)-1:-1]`. – Niklas B. Mar 30 '14 at 15:35
  • @NiklasB. Thanks. I didn't think of that. Fixed that in the answer. :) – Sukrit Kalra Mar 30 '14 at 16:08
  • @SukritKalra I got it! The begin minus step to get the index.And first the `len(l)` is the last item in list until the the index equal to the `-len(l)-1` out range of the list.So we get the reverse list. – lazybios Apr 04 '14 at 02:14
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some_list[start:end:step]

When you ommit any of the slicing operands they take on default values. Default values for start, end and step: start - the beginning of an indexated iterable which is always of an index 0 when step is positive, end - the ending index of an indexated iterable which is always its length (following the same convention as range) when step is positive, step - the default step is always one.

When you're using a minus sign on step omitting other operands you're basically saying "return a reversed list".

EDIT: Funny, but

[1,2,3,4,5,6][5:-7:-1]

returns the same result as

[1,2,3,4,5,6][::-1]

in Python3. Can anyone comment on to why? That means that default values of start and end actually rest upon the step operand (more specifically its sign).

Alisa D.
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    I think the idea is that `[start:end:step]` will visit the indices that would be visited by a loop like `for (idx = start; idx != end; idx += step)`. Negative indices `idx < 0` are interpreted as the "real" index `len(l) + idx`. `[5:-1:-1]` will not work because `-1` is interpreted as the index `len(l)-1`, so we need to use `-len(l)-1`, which is the "real" index -1 to force the iteration to include the index 0 – Niklas B. Mar 30 '14 at 15:38