[Python-Dev] RFC: PEP 509: Add a private version to dict

[Stefan Behnel]

Victor Stinner schrieb am 15.04.2016 um 00:33:
> 2016-04-15 0:22 GMT+02:00 Brett Cannon:
>> And even if it was GIL-free you do run the risk of two dicts ending up at
>> the same version # by simply mutating the same number of times if the
>> counters were per-dict instead of process-wide.
> 
> For some optimizations, it is not needed to check if the dictionary
> was replaced, or you check it directly. So it doesn't matter to have
> the same version with the same number of operations.
> 
> For the use case of Yury's optimization, having a globally unique
> version tag makes the guard much cheaper, and the guard must check
> that the dictionary was not replaced.

How can that be achieved? If the tag is just a sequentially growing number,
creating two dicts and applying one operation to the first one should give
both the same version tag, right?

Stefan