[Python-Dev] a feature i'd like to see in python #1: better iteration control

[Brian Harring]

On Mon, Dec 04, 2006 at 07:15:35PM +0100, "Martin v. L??wis" wrote:
> Brian Harring schrieb:
> > For dict; it actually *cannot* work.  You can't remove keys from a 
> > dict as you're iterating over it (can change the val of a key, but not 
> > remove the key).
> 
> I think this is incorrect. The implementation could well support it,
> putting a dummy object into the deleted key (which deletion needs
> to do, anyway).

The implementation already uses a sentinel (NULL)- point was that it 
does not support iteration over a dict that's being deleted from 
*currently* though.

One thing to note; delitem is the easy case.  Allowing for mutating 
the mapping as you're iterating via delitem implies that setitem 
should work also; setitem however can trigger a resize.

Finally, if dicts were ever modified to shrink based on load, the 
resize there would be an issue.  Mind you I've not seen proposals of 
that sort, just pointing out the potential.


> > So iter.delete would require fair bit of changes 
> > internally to dict, either tracking what it's yielded already, or 
> > forcing iterkeys to actually be iter(keys()) (creating an intermediate 
> > list), which is worse for memory usage and general performance.
> 
> I don't think either is necessary; deletion could occur "directly".
> 
> > Set's suffer the same thing; can't change what it contains while 
> > iterating, have to restart the iteration after a removal/addition.
> 
> Again, I think that's incorrect.

Again, was refering to existing implementation (and long standing 
rules/conventions regarding it).  Set suffers the same issue with 
setitem meanwhile.

In my opinion, no point in doing the deltitem modification without a 
matching setitem.  Not saying I think the modification is worth it 
mind you, just that there should be symmetry ;)


~harring
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