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

["Martin v. Löwis"]

Ben Wing schrieb:
> i do see your point.  i was trying to remember what i ended up doing 
> when i ran into this issue before, and in fact i ended up just making a 
> new list and appending all the elements i didn't want to delete.  i see 
> you'd get N^2 behavior if you deleted lots of elements from a list, but 
> you get the same thing currently if you call `del list[i]' a lot; it's 
> not obvious that iter.delete() actually "papers over" the cost any more 
> than `del list[i]' does.

No, it wouldn't. OTOH, copying into a new list and then performing
slice-assignment is O(N). So performance-wise, it is (surprisingly)
asymptotically better to create a new list.

> however, for hash tables there's no reason to use iter.delete().  you 
> should just be able to write `del hash[x]'.  is this disallowed 
> currently?  

Try for yourself:

py> x={1:2,3:4}
py> for k in x:
...   if k < 2:
...     del x[k]
...
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
RuntimeError: dictionary changed size during iteration

> if so, it seems like something that should be fixed.

It's difficult to fix: deleting an item may cause a resizing/rehashing
of the entire dictionary, hence the dictionary is looked while being
iterated over. I *think* that one could support deletion (but not
addition); this would need further investigation.

Regards,
Martin