Can you clarify what's going on with a comment?

Why is it important to get rid of empty strings in val? What about None?

The brief feedback for this diff is:
"if val dict contains empty strings '', filter them out from it, otherwise behave as before".
ie. I ride on the fact that current code cannot handle --amend=source=,tarball.tar.gz well and make a feature out of it.
If I'm not mistaken, this is what was proposed by you in an earlier message, no?

Indeed, this looks nicer, let's fix it:

>>> val=['hello', '']
>>> fval = [x for x in val if x]
>>> fval
['hello']

Your version is more general, i.e. it also filters out None, which shouldn't be there in the first place. So, it would hide bugs that introduced problems earlier... (sorry to be confusing)

OK, our off-line discussion with Ken revealed that the following would be more intuitive:

• starts with comma => prepends # ,hello.tgz
• end with comma => append # hello.tgz,
• else => overwrite # hello.tgz,world.tgz

IMHO, we hit a case alike: http://golang.org/doc/go1compat.html -> "Specification"
and we should be going to fix the comma behavior, agreed?

Fixed as asked, not yet tried in practice to make a comment on real-life results.

What's the logic about not including res.group(1) here? Shouldn't it be more something like:

fval = [x for x in val if x != '']
newval = "%s + %s" % (fval, res.group(1))

Ah, OK, now I see what's going on, it's about prepend vs overwrite... But the logic is reversed.

What we want is:

• starts with , => append to list
• ends with , => prepend to list
• else: overwrite

This breaks backward compatibility, but that's OK imho, since this is a rarely-depended on feature afaik.

Please fix indentation of comment.

use elif val[-1] == '' here