slicing lists

[Yves Dorfsman]

Miles wrote:
> On Wed, May 7, 2008 at 7:46 PM, Ivan Illarionov
>  >  > Is there a way to do:
>  >  > x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>  >  > x[0,2:6]
>  >  >
>  >  > That would return:
>  >  > [0, 3, 4, 5, 6]

Arg... Yes, this is a typo, I meant:
[1, 3, 4, 5, 6]

>  I think Yves meant to return [1, 3, 4, 5, 6], as in Perl's list slicing:
> 
>  my @x = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
>  return @x[0, 2..6]; // returns (1, 3, 4, 5, 6)

Yes, exactly.

> 
>  This isn't incredibly efficient, but it does what you want (I think):
> 
>  from itertools import chain
> 
>  class multisliceable(list):
>   def __getitem__(self, slices):
>     if isinstance(slices, (slice, int, long)):
>       return list.__getitem__(self, slices)
>     else:
>       return list(chain(*[list.__getitem__(self, s) if isinstance(s, slice)
>                           else [list.__getitem__(self, s)] for s in slices]))
> 
>  p = open('/etc/passwd')
>  q = [multisliceable(e.strip().split(':'))[0,2:] for e in p]

So would it be a worthy addition to python, to add it right in the core of 
the language, and hopefully in an efficient manner ?

That would certainly help some type of list comprehensions, making them more 
readable, and hopefully more readable (running split once instead of twice, 
or how many number of time you need it). The passwd example is just one 
example I ran into, but I can see running in this problem a lot with more 
complex cases. Right now I solve the passwd pb with:

p = file('/etc/passwd').readlines()
r = [ e.strip().split(':') for e in p ]
s = [  e[0:1] + e[2:]   for e in r ]


Or:

p = file('/etc/passwd').readlines()
s = [  e.strip().split(':')[0:1] + e.strip().split(':')[2:]   for e in p ]

In the first case we're looping twice (two list comprehension), in the 
second case we're running the split twice on every element of p.


Yves.
http://www.SollerS.ca