From: Edward K. Ream <edream...@gmail.com>
 To: leo-editor <leo-editor@googlegroups.com> 
 Sent: Friday, October 14, 2016 3:22 PM
 Subject: Re: reversed iterators
On Fri, Oct 14, 2016 at 1:42 PM, 'Terry Brown' via leo-editor 
<leo-editor@googlegroups.com> wrote:

Something that just struck me.  I want to add, to all the nodes on a 
multi-level subtree, two child nodes. Here's the code I used:
for nd in reversed([i for i in p.subtree_iter()]):    nd.insertAsLastChild().h 
= 'src'    nd.insertAsLastChild().h = 'clip'c.redraw()
I don't think that would have worked without the reversed(), maybe because 
positions would get corrupted (maybe not, after the recent addition of .copy() 
on iterator yields?), but definitely because it would end up recursively adding 
the pair of child nodes to themselves.
​Careful, the theory of operations for c.deletePositionsInList explains why 
there is no order for deleting a set of nodes. See Code-->Core classes-->@file 
leoCommands.py-->class Commands-->c.deletePositionsInList--><< theory of 
operation >> (deletePositionsInList)

Hmm, good point, I was only thinking of insertions. And of course, "Clones 
Spoil Everything"(TM), so I'm sure there are traps there too.  Still, for my 
use case, reversed() certainly seems like the simplest way, otherwise you'd 
need to collect gnxs or something.
Cheers -Terry

It's hard to remember the argument, but the conclusion is engraved in my bean:  
"the conclusion...is we must use the positions passed to 
p.deletePositionsInList only as *hints* about what to do."

Reversed iterators may be safe in some situations, but usually they are just as 
unsafe as forward iterators. So it seems that providing reversed iterators is 
going to give people a false sense of security.

Edward
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