help with simple script and strings

[Kyle VanderBeek]

On Sat, Apr 12, 2008 at 01:35:33PM -0700, John Poelstra wrote:
> directories = ['20080412', '20080324', 'blahblah', 'latest-dir', 
> 'rawhide-20080410', 'rawhide-20080411', 'rawhide-20080412' , '20080401']
> 
> print 'directories == %s' % directories
> 
> for directory in directories:
>     print 'processing directory %s ' % directory
>     if not directory.startswith('200'):
>         directories.remove(directory)
>         print 'removed == %s' % directory
> 
> print 'directories == %s' % directories

Modifying the contents of a list that you're iterating over with a 
generator will give you strange results.  Basically, you're causing the 
generator to lose its place.  If you put the result in a new list (say, 
directories_new), this won't happen.

Granted, I'd probably do it this way, just going once over the list with 
a list iteration:

>>> [d for d in directories if d.startswith('200')]
['20080412', '20080324', '20080401']

Since it builds a new list, you can even assign it back to the 
directories variable:

>>> directories = [d for d in directories if d.startswith('200')]
>>> directories
['20080412', '20080324', '20080401']

I'm not a huge fan of list comprehensions due to the somewhat baroque
syntax, but this seems like a perfect single-pass O(n) application of
the construct that is still easy to read.

-- 
kylev at kylev.com
  Some people have a way with words, while others... erm... thingy.