set and dict iteration

[Aaron Brady]

On Thursday, August 16, 2012 8:01:39 PM UTC-5, Paul Rubin wrote:
> Ian Kelly <ian.g.kelly at gmail.com> writes:
> 
> > With regard to key insertion and deletion while iterating over a dict
> 
> > or set, though, there is just no good reason to be doing that
> 
> > (especially as the result is very implementation-specific), and I
> 
> > wouldn't mind a more complete low-level check against it as long as
> 
> > it's not too expensive (which is not clearly the case with the current
> 
> > suggestion at all).
> 
> 
> 
> One possible approach is to freeze the dictionary against modification
> 
> while any iterator is open on it.  You could keep a count of active
> 
> iterators in the dict structure, adjusting it whenever an iterator is
> 
> created or closed/destroyed.

Good point.  Your approach is another consistent solution.

The difference is in where the exception is raised.  Invalidating the iterators raises an exception when they're called.  Locking the set/dict raises an exception on 'add' and 'remove' calls.

The latter also forces additional statements to delete iterators before they leave scope in some cases.  We wouldn't be able to make modifications in a 'for' suite, even if followed by a 'break', which could be a problem for existing code.