[ python-Bugs-1303928 ] Extended slice bug (or feature?)

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Bugs item #1303928, was opened at 2005-09-25 13:06
Message generated for change (Comment added) made by beazley
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Category: Python Interpreter Core
Group: Python 2.4
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: David M. Beazley (beazley)
Assigned to: Nobody/Anonymous (nobody)
Summary: Extended slice bug (or feature?)

Initial Comment:
Consider a sequence:

a = [1,2,3,4,5,6,7,8,9,10]

Now, consider the following slices:

b = a[:-5]        # b gets [1,2,3,4,5]
c = a[:-5:1]     # c gets [1,2,3,4,5]
d = a[:-5:-1]    # d gets [10,9,8,7]

Is this the intended behavior??  I would have suspected the value of 
d to be [5,4,3,2,1]  (the same elements of c, but in reverse order).

Thanks.  -- Dave


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>Comment By: David M. Beazley (beazley)
Date: 2005-09-25 15:50

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Thanks for the clarification----just making sure I get this absolutely right 
before committing anything to paper here.  -- Dave.

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Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-09-25 13:25

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Closing again ;)

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Comment By: Michael Hudson (mwh)
Date: 2005-09-25 13:17

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Yes, it's intended.  l[i:j:k][0] == l[i] when it makes sense, and the sense of 
omitted indices flips depending on the sign of k.  It's a bit confusing, but 
then I think all the variants are in some cases.



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Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-09-25 13:16

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No. With a step of -1, the slice always starts at the end
(at 10). The stop (that is 6, in this case) is never
included, and so you get your result.

If you want [5,4,3,2,1], use a[:-5][::-1] or reversed(a[:-5]).

PS: comp.lang.python is a better place to discuss whether
something is really a bug.

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