Negative array indicies and slice()

[Andrew Robinson]

On 10/29/2012 06:52 AM, Roy Smith wrote:
> Show me an example where someone would write a slice with a negative and
> a positive index (both in the same slice);
> and have that slice grab a contiguous slice in the *middle* of the list
> with orientation of lower index to greater index.
> It's possible in bioinformatics. ...
> eq[100:-100].
I decided to go to bed... I was starting to write very badly worded 
responses. :)

Thanks, Roy,  what you have just shown is another example that agrees 
with what I am trying to do.
FYI: I was asking for a reason why Python's present implementation is 
desirable...

I wonder, for example:

Given an arbitrary list:
a=[1,2,3,4,5,6,7,8,9,10,11,12]

Why would someone *want* to do:
a[-7,10]
Instead of saying
a[5:10] or a[-7:-2] ?

eg:
What algorithm would naturally *desire* the default behavior of slicing 
when using *mixed* negative and positive indexes?
In the case of a bacterial circular DNA/RNA ring, asking for codons[ 
-10: 10 ]  would logically desire codon[-10:] + codon[:10] not an empty 
list, right?

I think your example is a very reasonable thing the scientific community 
would want to do with Python.
:)