Problem with OrderedDict

[Chris Angelico]

On Wed, May 30, 2018 at 10:55 PM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> On Wed, 30 May 2018 05:03:02 -0400, Terry Reedy wrote:
>
>> On 5/30/2018 4:48 AM, Frank Millman wrote:
>>> Hi all
>>>
>>> I want to work backwards to solve this problem, so I have to explain it
>>> forwards to put you in the picture.
>>>
>>> I have an Ordered Dictionary. Under certain circumstances I am getting
>>> this error -
>>>
>>>     RuntimeError: OrderedDict mutated during iteration
>>
>> This should mean that the value associated with a key was sometimes
>> changed.
>
> I don't think so. It should be legal to iterate over a dict and change
> the values. At least it works for me:
>
> import random
> from collections import OrderedDict
> d = OrderedDict(zip(range(1, 10), "abcdefghi"))
> for i in range(10000):  # just in case of intermittent errors
>     for value in d.values():
>         d[random.randrange(1, 10)] = random.random()
>
>
> I get no errors.
>

You can always change the *values*, but not the *order* of the keys.

>>> from collections import OrderedDict
>>> d = OrderedDict(zip(range(1, 10), "abcdefghi"))
>>> for x in d:
...   if x == 5: d.move_to_end(x)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: OrderedDict mutated during iteration

ChrisA