RuntimeError: dictionary changed size during iteration

[Robert Dailey]

On Dec 8, 10:27 pm, John Machin <sjmac... at lexicon.net> wrote:
> On Dec 9, 3:00 pm, Steven D'Aprano
>
>
>
> <ste... at REMOVE.THIS.cybersource.com.au> wrote:
> > On Mon, 08 Dec 2008 19:10:00 -0800, Robert Dailey wrote:
> > > On Dec 8, 6:26 pm, Terry Reedy <tjre... at udel.edu> wrote:
> > >> Robert Dailey wrote:
> > >> > stuff = vars()
>
> > >>  >>> vars() is globals()
> > >> True
>
> > >> > for key in stuff:
>
> > >> You just changed globals, which is aliased as stuff. Stuff changes.
>
> > >> >     print( key, '--', stuff[key] )
>
> > >> > I get the following error message:
> > >> > ('CopyEmotionFX', '--', <function CopyEmotionFX at 0x0205BF70>)
> > >> > Traceback (most recent call last):
> > >> >   File "C:\IT\work\jewett\depends.py", line 12, in <module>
> > >> >     for key in stuff:
> > >> > RuntimeError: dictionary changed size during iteration
>
> > >> > Why is this happening?
>
> > > How am I changing globals()? I'm simply iterating the keys in the dict.
> > > Can someone explain what is going on please?
>
> > You create an new name "key":
>
> > for key in stuff
>
> > I suppose you could do this:
>
> > key = None
> > stuff = vars()
> > for key in stuff:
>
> but both 'key' and 'stuff' will appear in the dict, possibly causing
> confusion; another reason why
>
> > even better would be:
>
> > for key in vars().copy():
>
> > because that protects you from cases where globals() change inside the
> > for loop.
>
>

When I do:

for key in stuff.keys():


It works! I wonder why .keys() makes a difference. It is using a
'view', which is a new concept in Python 3.0 that I'm not totally
familiar with yet.