It's certainly a minor thing (the possible size reduction is 15 Bytes), but I find it unfortunate if I open a scoured file and the first thing I see is an empty defs.
It happens like this: I have a default svg template file for Inkscape which contains some standard gradients I may or may not be using while creating content. I may end up saving a file where none of them is referenced. Next I start scour.
Scour looks for empty defs', but my defs contains some gradients, so it leaves my defs untouched. Next, it checks for unreferenced gradients, and since all my gradients are unreferenced, it removes them all, as it should. Now it saves the file, with a defs element which is now empty.
I provided a patch which, hopefully, solves this.
Within scourString(), I swapped the order of the removal of empty defs and the call of removeUnreferencedElements(), but that wasn't sufficient: the removal of empty defs is also the same code as the removal of empty metadata and the removal of empty groups. After successfully removing gradients, filters, pattern and symbols from my defs element, those removed elements left some text nodes behind which contain some whitespace. When checking for the presence of children, scour also checks for the presence of text node children, since it preserves metadata with (nothing but) text (it checks for the presence of child nodes of type 1, 3, 4 or 8, with 3 being the type of text nodes).
I handled this by checking whether a child text node contains nothing but whitespace characters; a metadata element with nothing but whitespace as child nodes should be something we should be allowed to delete.
While I was at it, I also tried to slightly update the description of removeUnreferencedElements(), and deleted setting num=0 a second time halfway through the code, which, I think, shouldn't be there. Within scourString(), I deleted a variable elemsToRemove which doesn't do anything.
Finally, I added a new file for unittest with a defs element with nothing but unreferenced removable children and added a test to testscour.py to see whether it gets removed.
Thanks Jan - this looks good to me, including the addition of the unit test, I assume all the other tests pass.
Since Louis has been much more involved with the project lately I'll let him review and merge it in to the trunk.
Thanks for your contribution!