pickle behavior

[ToddW]

When pickling an object, is the expected behavior that it be pickled
identically each and every time? Ie. I want to know if I can
rely on the string generated always being the same given the object
can be represented identically (for example tuples are immutable,
and therefore should be able to be pickled identically in my view).

As far as I can tell (and I have tested fairly extensively), this is
the behavior of pickle, but not cpickle.

Can anyone clarify the proper behavior? Is the cpickle anomoly NOTABUG?