[Python-bugs-list] [ python-Bugs-684022 ] extended slice strangeness

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Bugs item #684022, was opened at 2003-02-10 16:54
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Category: Python Interpreter Core
Group: Python 2.3
>Status: Closed
>Resolution: Rejected
Priority: 5
Submitted By: Robert Roy (rjroy)
Assigned to: Michael Hudson (mwh)
Summary: extended slice strangeness

Initial Comment:
I was playing with extended slices and wanted to see if 
it would return a reversed slice. The behaviour was not 
quite what I expected. See the following


Python 2.3a1 (#38, Dec 31 2002, 17:53:59) [MSC 
v.1200 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more 
information.
>>> l = range(12)
>>> l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
## this works as expected
>>> l[0:5:2]
[0, 2, 4]

## this returns empty list ??
>>> l[0:5:-1]
[]

## this works as expected
>>> l[::-1]
[11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

## I would expect this to return same as above
>>> l[0::-1]
[0]

## would expect empty list since l[5:0] == []
>>> l[5:0:-1]
[5, 4, 3, 2, 1]

## would expect [11, 10, 9, 8, 7, 6,5]
>>> l[5::-1]
[5, 4, 3, 2, 1, 0]
>>>


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>Comment By: Michael Hudson (mwh)
Date: 2003-02-10 17:50

Message:
Logged In: YES 
user_id=6656

the interpretation of 

l[start:stop:step]

is similar to that of 

range(start, stop, end)

An omitted an index means the endpoint and what that means
changes with the sign of step.  You have to have this to
make l[::-1] do what you want.

So I'm going to reject this.  Feel free to reopen if you
disagree wildly, but you'll have to talk fast.

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Comment By: Neal Norwitz (nnorwitz)
Date: 2003-02-10 17:12

Message:
Logged In: YES 
user_id=33168

I'm not sure what I would expect.  Your suggestions seem
reasonable.  Michael?

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