[Python-Dev] removing inst_persistent_id from pickle

[Jeremy Hylton]

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?

Jeremy