...

write a small script (python),

get_index try: my_index = context.index except: my_index = 0 return my_index

...

now with <tree .. sort=get_index>

I did it with plain DTML:

<dtml-if "_.hasattr(aq_explicit,'dirIndex')"><dtml-return dirIndex> <dtml-else><dtml-return "0"> </dtml-if>

but didn't work. No error, but also no correct sorting. The order of things is different from that when using no sort= attribute, but I couldn't figure out what kind of order it is :-(

Even when I just return the id in my dtml method, where you would expect tree-tag to sort things by id, it doesn't do the sort correctly.

So, anybody an idea what's going on here?

First, obvious question, why don't you try the python script? Just tried it, it works (for me). As for your dtml-approach, no idea why it doesn't work, but I can indeed confirm this. Perhaps a sign? "Thou shalt not abuse dtml for pure logic, for it will cometh back and bite you" ;-) cheers, oliver