Separating elements from a list according to preceding element

[James Stroud]

Rob Cowie wrote:
> I'm having a bit of trouble with this so any help would be gratefully
> recieved...
> 
> After splitting up a url I have a string of the form
> 'tag1+tag2+tag3-tag4', or '-tag1-tag2' etc. The first tag will only be
> preceeded by an operator if it is a '-', if it is preceded by nothing,
> '+' is to be assumed.
> 
> Using re.split, I can generate a list that looks thus:
> ['tag1', '+', 'tag2', '+', 'tag3', '-', 'tag4']
> 
> I wish to derive two lists - each containing either tags to be
> included, or tags to be excluded. My idea was to take an element,
> examine what element precedes it and accordingly, insert it into the
> relevant list. However, I have not been successful.
> 
> Is there a better way that I have not considered? If this method is
> suitable, how might I implement it?
> 
> Thanks all,
> 
> Rob Cowie
> 

Unclever way:

alist = ['tag1', '+', 'tag2', '+', 'tag3', '-', 'tag4']
include, disinclude = [], []
aniter = iter(alist)
if len(alist) % 2:
   include.append(aniter.next())
for asign in aniter:
   if asign == '+':
     include.append(aniter.next())
   else:
     disinclude.append(aniter.next())

A cleverer way will probably use list comprehension and logic shortcutting.

James