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

[Yury Selivanov]

On 2016-01-20 2:02 PM, Maciej Fijalkowski wrote:
> On Wed, Jan 20, 2016 at 8:00 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
>
[..]
>>> Brett, you need two things - the ID of the dict and the version tag.
>>> What we do in pypy is we have a small object (called, surprisingly,
>>> VersionTag()) and we use the ID of that. That way you can change the
>>> version id of an existing dict and have only one field.
>> Yeah, that's essentially what I propose with ma_extra.
>>
>> Yury
> The trick is we use only one field :-)
>
> you're proposing to have both fields - version tag and dict id. Why
> not just use the id of the object (without any fields)?

What if your old dict is GCed, its "VersionTag()" (1) object is
freed, and you have a new dict, for which a new "VersionTag()" (2)
object happens to be allocated at the same address as (1)?

Yury