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I agree that this a bug. However, the liberal functools.partial
constructor makes it hard to pickle partial instances correctly.
Ideally, we would add the __getnewargs__ special method and be done with
it. But, this won't work in this case due to the *args and **kwargs
arguments of partial. Since pickle supports neither, we would need to
use apply(), which going to be removed in Python 3.0, as follow:
>>> import pickletools
>>>
pickletools.dis("c__builtin__\napply\n(cfunctools\npartial\n(c__main__\nf\nt(S'b'\nK\x01dtR.")
 0: c GLOBAL '__builtin__ apply'
 19: ( MARK
 20: c GLOBAL 'functools partial'
 39: ( MARK
 40: c GLOBAL '__main__ f'
 52: t TUPLE (MARK at 39)
 53: ( MARK
 54: S STRING 'b'
 59: K BININT1 1
 61: d DICT (MARK at 53)
 62: t TUPLE (MARK at 19)
 63: R REDUCE
 64: . STOP
Unfortunately, pickle.Pickler cannot generate a such pickle stream. So
this bug is symptom of the bigger issue that classes with *args and/or
**kwargs argument cannot be made picklable.