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

[Maciej Fijalkowski]

On Wed, Jan 20, 2016 at 8:08 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
>
> 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
>

You don't free a version tag that's stored in the guard. You store the
object and not id