Problem with OrderedDict - progress report

[Steven D'Aprano]

On Thu, 31 May 2018 16:37:39 +0200, Frank Millman wrote:

[...]
> Agreed, but my gut feel, and the following example, suggest that when
> processing the last key in a dictionary while iterating over it, you
> have not yet stopped iterating.
> 
>>>> d = {}
>>>> d[1] = 'one'
>>>> d[2] = 'two'

Before Python 3.7, regular dicts are unordered. Just because you add 2 
last, doesn't mean that 2 is the last item. The position of each item is 
unpredictable, and 2 could end up in the first position.

In this example:


>>>> for k in d:
> ...   if k == 2:
> ...     d[3] = 'three'
> ...
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> RuntimeError: dictionary changed size during iteration

just because k == 2 does not mean this matches on the last iteration. It 
could be the first.


> OrderedDict seems to behave differently in this regard -

Neither OrderedDict nor dict guarantee to detect all mutations.


-- 
Steven D'Aprano
"Ever since I learned about confirmation bias, I've been seeing
it everywhere." -- Jon Ronson