I was attempting the patch for issue3125 to py3k, and in it Amaury 
defines a new ForkingPickler:

from pickle import Pickler
class ForkingPickler(Pickler):
    dispatch = Pickler.dispatch.copy()

This is also related to issue3350 I suspect. However, using the 
pickle.Pickler module under py3k, there is no dispatch() method on the 
class:

Trunk:
Python 2.6b1+ (trunk:65015M, Jul 16 2008, 10:15:51) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> dir(pickle.Pickler)
['_BATCHSIZE', '__doc__', '__init__', '__module__', '_batch_appends', 
'_batch_setitems', 'clear_memo', 'dispatch', 'dump', 'get', 'memoize', 
'persistent_id', 'put', 'save', 'save_bool', 'save_dict', 
'save_empty_tuple', 'save_float', 'save_global', 'save_inst', 
'save_int', 'save_list', 'save_long', 'save_none', 'save_pers', 
'save_reduce', 'save_string', 'save_tuple', 'save_unicode']
>>> 

py3k:
Python 3.0b1+ (py3k:65011M, Jul 16 2008, 11:50:11) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> dir(Pickler)
['__class__', '__delattr__', '__doc__', '__eq__', '__format__', 
'__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', 
'__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', 
'__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 
'bin', 'clear_memo', 'dump', 'fast', 'memo', 'persistent_id']

I think the fix for 3125 resolves your complaint in 3350, but is the 
lack of the dispatch method intentional?

The omission of the dispatch dictionary was sort of intentional. But 
now, I think it would be a good idea to add it to _pickle.Pickler. I 
will write a patch ASAP.

Just in case you are wondering why I haven't submitted a patch yet, I 
want to let you know that my home computer is currently broken. So, I 
won't be able to work on this until I get my computer fixed (which 
unfortunately could take a few weeks).

A use case:
Python/Recipe/572213">http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/572213
shows how code can use the Pickler.dispatch dict, but also some
Pickler.save_xxx function which should be exposed as well.

I got a preliminary patch that adds the dispatch dictionary. However,
the patch leaks and it doesn't expose the save_reduce() method (which is
needed by ForkingPickler).

I ran into a few problems while trying to fix this issue. First, does
someone know how to add class attributes on extension types? It sounds
like I will need either some tp_dict hacking or a Pickler subclass.

Second, which methods of Pickler should be made public? I know
save_reduce() is needed, but would it be worthwhile to expose more? In
the recipe Amaury linked (which abuses Pickler IMHO), save_global(),
save_dict(), write() and memoize() are used. Exposing all save_* methods
is out of question for now as none were written to be used standalone.

Third, should Pickler allows pickling support for built-in types (e.g.,
int, dict, tuple, etc) to be overridden? Currently, pickle.py allows it.
However, I am not sure if it is a good idea to copy this "feature" in
_pickle.c.

Maybe I missed something, but the (subclassable) python implementation
of Pickler is still available with another name:

from pickle import _Pickler as Pickler
class ForkingPickler(Pickler):
    dispatch = Pickler.dispatch.copy()

Yeah, the old Pickler and Unpickler classes are available by design (to
allow testing both implementation). You could subclass _Pickler as a
temporary fix for this issue.