[Python-Dev] Re: removing inst_persistent_id from pickle

[Jim Fulton]

Jeremy Hylton wrote:
> Pickle and cPickle have an undocumented hook called
> inst_persistent_id().  When Barry updated the pickle documentation
> (round about the 2.2 release), no one could figure out what it did.  I
> think we've figured out what it does, finally, but I think we should
> get rid of it before someone tries to use it.
> 
> The pickler has a hook for handling persistent objects.  The hook
> basically allows objects to be pickled by reference to some mechanism
> external to the pickler.  The pickler gets a persistent_id() function
> that returns the external reference.  The unpickler needs a
> persistent_load() function that returns the object given the reference
> from persistent_id().  This process is fairly general, although the
> only use I'm familiar with is ZODB.
> 
> The inst_persistent_id() hook seems to be designed for a very special
> case -- that the persistent_id() function returns an object that is
> unpicklable.  The function is only called when the pickler encounters
> an object that it hasn't handled via persistent_id() or the dispatch
> table.  The object returned by inst_persistent_id() is always passed
> to save_pers(), just like persistent_id().
> 
> We imagine the intended control flow is: 
> 
>   - pickler created in binary mode
>   - persistent_id() returns an unpicklable object
>   - inst_persistent_id() is called to convert this to a picklable
>     object
> 
> I don't think this odd case is worth the added complexity,
> particularly since the hook function probably won't be called for a
> text-mode pickler.
> 
> Anyone object to its removal?

No. In fact, I endorse it's removal.

Jim


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