[Python-Dev] Pickler/Unpickler API clarification

[Michael Haggerty]

Greg Ewing wrote:
> Michael Haggerty wrote:
>> A similar effect could *almost* be obtained without accessing the memos
>> by saving the pickled primer itself in the database.  The unpickler
>> would be primed by using it to load the primer before loading the record
>> of interest.  But AFAIK there is no way to prime new picklers, because
>> there is no guarantee that pickling the same primer twice will result in
>> the same id->object mapping in the pickler's memo.
> 
> Would it help if, when creating a pickler or unpickler,
> you could specify another pickler or unpickler whose
> memo is used to initialise the memo of the new one?
> 
> Then you could keep the pickler that you used to pickle
> the primer and "fork" new picklers off it, and similarly
> with the unpicklers.

Typically, the purpose of a database is to persist data across program
runs.  So typically, your suggestion would only help if there were a way
to persist the primed Pickler across runs.

(The primed Unpickler is not quite so important because it can be primed
by reading a pickle of the primer, which in turn can be stored somewhere
in the DB.)

In the particular case of cvs2svn, each of our databases is in fact
written in a single pass, and then in later passes only read, not
written.  So I suppose we could do entirely without pickleable Picklers,
if they were copyable within a single program run.  But that constraint
would make the feature even less general.

Michael