mutating a deque whilst iterating over it

[duncan smith]

On 13/02/2021 19:12, Dan Stromberg wrote:
> On Sat, Feb 13, 2021 at 10:25 AM duncan smith <duncan at invalid.invalid>
> wrote:
> 
>> On 12/02/2021 03:04, Terry Reedy wrote:
>>> On 2/11/2021 3:22 PM, duncan smith wrote:
>>>
>>>>        It seems that I can mutate a deque while iterating over it if I
>>>> assign to an index, but not if I append to it. Is this the intended
>>>> behaviour?
>>
>> What I was really wondering was whether the behaviour is as it is
>> because of the implementation or because it's how deques should ideally
>> behave. i.e. In my implementation do I stick strictly to the same API,
>> or allow it to differ? In some places I'm jumping through hoops to stick
>> to the API, and (when it comes to iteration) I'm jumping through
>> different hoops for different types of container (e.g. lists versus
>> deques). BTW, the reason I am implementing these at all is that my
>> containers are on-disk. Cheers.
>>
>> collections.deque appears to take the approach of  "allow everything we
> can based on our implementation, and trust the client not to overuse
> features".
> 
> In fact, in Python, "over abstraction" is often seen as a bad thing, mostly
> because it slows things down so much.
> 
> If you want something more abstracted, you might have a look at:
> https://stromberg.dnsalias.org/~strombrg/linked-list/
> https://pypi.org/project/linked_list_mod/
> 
> (Both URL's are about the same module)
> 
> Note that linked_list_mod is quite a bit slower than a collections.deque.
> Deques use a clever-but-hidden trick to gain a lot of speed - it's really a
> linked list of built in lists, which gives better locality of reference
> that masks CPU caches very happy.  linked_list_mod goes for abstraction and
> simplicity.
> 

I'm basically nicking the trick. I have a basic, single file, on-disk
deque class that doesn't stick strictly to the Python deque API, then a
second class that implements a deque as a Python deque containing
instances of my basic on-disk deque class (with fixed size apart from
the first and last instances). Of course, this could change when I get
to testing / profiling. Cheers.

Duncan