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

Brian Harring ferringb at gmail.com
Tue Dec 5 01:51:59 CET 2006

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 ;)

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