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

[Glenn Linderman]

On 1/20/2016 4:08 PM, Brett Cannon wrote:
>
>
> On Wed, 20 Jan 2016 at 15:46 Victor Stinner <victor.stinner at gmail.com 
> <mailto:victor.stinner at gmail.com>> wrote:
>
>     Hi,
>
>     2016-01-20 22:18 GMT+01:00 Glenn Linderman <v+python at g.nevcal.com
>     <mailto:v%2Bpython at g.nevcal.com>>:
>     > On 1/20/2016 12:50 PM, Brett Cannon wrote:
>     >>
>     >> A global (shared between all dicts) unit64 ma_version is
>     actually quite
>     >> reliable -- if a program does 1,000,000 dict modifications per
>     second,
>     >> it would take it 600,000 years till wrap-around.
>
>     I think that Yury found a bug in FAT Python. I didn't test the case
>     when the builtins dictionary is replaced after the definition of the
>     function. To be more concrete: when a function is executed in a
>     different namespace using exec(code, namespace). That's why I like the
>     PEP process, it helps to find all issues before going too far :-)
>
>     I like the idea of global counter for dictionary versions. It means
>     that the dictionary constructor increases this counter instead of
>     always starting to 0.
>
>     FYI a fat.GuardDict keeps a strong reference to the dictionary. For
>     some guards, I hesitated to store the object identifier and/or using a
>     weak reference. An object identifier is not reliable because the
>     object can be destroyed and a new object, completly different, or of
>     the same type, can get the same identifier.
>
>     > But would invalidate everything, instead of just a fraction of
>     things, on
>     > every update to anything that is monitored...
>
>     I don't understand this point.
>
>
> I think Glenn was assuming we had a single, global version # that all 
> dicts shared *without* having a per-dict version ID. The key thing 
> here is that we have a global counter that tracks the number of 
> mutations for *all* dictionaries but whose value we store as a 
> *per-dictionary* value. That ends up making the version ID inherently 
> both a token representing the state of any dict but also the 
> uniqueness of the dict since no two dictionaries will ever have the 
> same version ID.

This would work. You were correct about my assumptions.
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