Cameron Simpson <c...@cskk.id.au> wrote: > On 23Aug2020 10:00, Chris Green <c...@isbd.net> wrote: > >I have a (fairly) simple little program that removes old mail messages > >from my junk folder. I have just tried to upgrade it from Python 2 to > >Python 3 and now, when it finds any message[s] to delete it produces > >the error:- > > > > RuntimeError: dictionary changed size during iteration > > > >I can sort of see why I'm getting the error but have two questions: > > > >1 - Why doesn't it error in Python 2? > > The dict internal implementation has changed. I don't know the > specifics, but it is now faster and maybe smaller and also now preserves > insert order. > Ah, that probably explains it then.
> >2 - How do I fix it? > > That standard way is to take a copy of what you're iterating over, > usually just the keys (consumes less memory). > > BTW, this approach is standard for _any_ data structure you're iterating > over and modifying. > > So: > > for k, v in d.iteritems(): > ... d.remove(k) ... > > might become: > > for k, v in list(d.iteritems()): > ... d.remove(k) ... > I've been [not]understanding it the wrong way round! I was somehow trying to copy the iterator and then doing things to the copy, I now see that you make a copy to iterate over (that isn't modified) and you do what's necessary to the original. Obvious really! :-) -- Chris Green ยท -- https://mail.python.org/mailman/listinfo/python-list
