Problem with OrderedDict

[Terry Reedy]

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 can see why this is happening, and I am pretty sure I can fix it by 
> making a copy to iterate over.
> 
> However, the error seems to be intermittent - there are times when it 
> should fail, but does not - so I want to investigate further.
> 
> I tried to reduce it to a simple example. I succeeded, but there are two 
> problems -
> 
> 1. It always fails, so I have not reproduced the intermittent nature yet.
> 
> 2. It gives a different error -
> 
>     RuntimeError: dictionary changed size during iteration

This should mean that a new key-value pair was always added or removed.

> So my first question is, what is the difference between the two error 
> messages? I am using an OrderedDict for my test as well, so the 
> difference is not caused by using a normal dictionary.
> 
> I am using Python 3.6.0.

-- 
Terry Jan Reedy