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If you pickle an array object on python 3 the typecode is encoded as a unicode string rather than as a byte string. This makes python 2 reject the pickle.
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Python 3.3.0a0 (default, Dec 8 2011, 17:56:13) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle, array
>>> pickle.dumps(array.array('i', [1,2,3]), 2)
b'\x80\x02carray\narray\nq\x00X\x01\x00\x00\x00iq\x01]q\x02(K\x01K\x02K\x03e\x86q\x03Rq\x04.'
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Python 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pickle
>>> pickle.loads(b'\x80\x02carray\narray\nq\x00X\x01\x00\x00\x00iq\x01]q\x02(K\x01K\x02K\x03e\x86q\x03Rq\x04.')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "c:\Python27\lib\pickle.py", line 1382, in loads
 return Unpickler(file).load()
 File "c:\Python27\lib\pickle.py", line 858, in load
 dispatch[key](self)
 File "c:\Python27\lib\pickle.py", line 1133, in load_reduce
 value = func(*args)
TypeError: must be char, not unicode