__reduce__ gets used by pickle and cPickle, when defined by regular Python classes

gistfile1.txt

>>> import pickle
>>> class C(object):
...     def __init__(self, banana):
...             self.banana = banana
...     def __reduce__(self):
...             return (C, (self.banana,))
... 
>>> pickle.dumps(C(1))
'c__main__\nC\np0\n(I1\ntp1\nRp2\n.'
>>> pickle.loads(pickle.dumps(C(1)))
<__main__.C object at 0x7eff0efa9ed0>
>>> pickle.loads(pickle.dumps(C(1))).banana
1
>>> import cPickle
>>> cPickle.dumps(C(1))
'c__main__\nC\np1\n(I1\ntRp2\n.'
>>> cPickle.loads(cPickle.dumps(C(1)))
<__main__.C object at 0x7eff0efa9f90>
>>> cPickle.loads(cPickle.dumps(C(1))).banana
1

and to show that it definitely is getting called:

>>> class C(object):
...     def __init__(self, banana):
...             print "constructing", banana
...             self.banana = banana
...     def __reduce__(self):
...             print "reducing", self.banana
...             return (C, (self.banana,))
...
>>> import pickle, cPickle
>>> pickle.dumps(C(1))
constructing 1
reducing 1
'c__main__\nC\np0\n(I1\ntp1\nRp2\n.'
>>> pickle.loads(pickle.dumps(C(1)))
constructing 1
reducing 1
constructing 1
<__main__.C object at 0x7fe558858d50>