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To: python-list@python.org
From: Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de>
Subject: Re: Strange behavior for a 2D list
Date: Thu, 18 Apr 2013 21:19:00 +0000 (UTC)
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Xref: csiph.com comp.lang.python:43861

Robrecht W. Uyttenhove <ruyttenhove <at> gmail.com> writes:

> 
> Hello,
> I tried out the following
code:y=[range(0,7),range(7,14),range(14,21),range(21,28),range(28,35)]
> >>> y[[0, 1, 2, 3, 4, 5, 6],  [7, 8, 9, 10, 11, 12, 13], 
>       [14, 15, 16, 17, 18, 19, 20],  [21, 22, 23, 24, 25, 26, 27],  [28, >
       29, 30, 31, 32, 33, 34]]
> >>> y[1:5:2][::3]
> [[7, 8, 9, 10, 11, 12, 13]]
> I expected the 2D list:[[ 7, 10, 13],
>  [21, 24, 27]]
> 
> Any ideas?
> 
> Thanks,
> Rob
> PS: I used Python 2.7.3
> 

The explanation is rather simple, just break up your complex slicing into
its parts:
y[1:5:2] => [[7, 8, 9, 10, 11, 12, 13],[21, 22, 23, 24, 25, 26, 27]]
and [::3] is asking for the first,4th,7th,... element from this list.
Obviously, only the first one's existing, so [7, 8, 9, 10, 11, 12, 13]

What you expected is kind of vertical slicing through the rows. I don't
think you can achieve this with slicing alone in standard Python, but it's
possible with numpy arrays.
In Python you will have to combine slicing with a comprehension, like this:
[x[::3] for x in y[1:5:2]]

Best,
Wolfgang