[Python-ideas] RFC: PEP: Add dict.__version__

[Steven D'Aprano]

On Sat, Jan 09, 2016 at 01:09:13PM +0100, M.-A. Lemburg wrote:

> * Given that this is an optimization and not meant to be exact
>   science, why would we need 64 bits worth of version information ?
>
>   AFAIK, you only need the version information to be able to
>   answer the question "did anything change compared to last time
>   I looked ?".
>
>   For an optimization it's good enough to get an answer "yes"
>   for slow changing dicts and "no" for all other cases.

I don't understand this. The question has nothing to do with 
how quickly or slowly the dict has changed, but only on whether or not 
it actually has changed. Maybe your dict has been stable for three 
hours, except for one change; or it changes a thousand times a second. 
Either way, it has still changed.


>   False
>   negatives don't really hurt. False positives are not allowed.

I think you have this backwards. False negatives potentially will 
introduce horrible bugs. A false negative means that you fail to notice 
when the dict has changed, when it actually has. ("Has the dict 
changed?" "No.") The result of that will be to apply the optimization 
when you shouldn't, and that is potentially catastrophic (the entirely 
wrong function is mysteriously called).

A false positive means you wrongly think the dict has changed when it 
hasn't. ("Has the dict changed?" "Yes.") That's still bad, because you 
miss out on the possibility of applying the optimization when you 
actually could have, but it's not so bad. So false positives (wrongly 
thinking the dict has changed when it hasn't) can be permitted, but 
false negatives shouldn't be.


>   What you'd need to answer the question is a way for the
>   code in need of the information to remember the dict
>   state and then later compare it's remembered state
>   with the now current state of the dict.
> 
>   dicts could do this with a 16-bit index into an array
>   of state object slots which are set by the code tracking
>   the dict.
> 
>   When it's time to check, the code would simply ask for the
>   current index value and compare the state object in the
>   array with the one it had set.

If I've understand that correctly, and I may not have, that will on 
detect (some?) insertions and deletions to the dict, but fail to 
detect when an existing key has a new value bound.


-- 
Steve