New submission from Eugene Toder <elto...@gmail.com>:
Pickle documentation [1] says:
""" Note: If __getstate__() returns a false value, the __setstate__() method
will not be called upon unpickling. """
However, this isn't quite true. This depends on the version of pickle protocol.
A small example:
>>> class Pockle(object):
def __getstate__(self):
return 0
def __setstate__(self, state):
sys.stdout.write('__setstate__ is called!\n')
>>> for p in range(4):
sys.stdout.write('protocol %d: ' % p)
pickle.loads(pickle.dumps(Pockle(), p))
protocol 0: <__main__.Pockle object at 0x0000000002EAE3C8>
protocol 1: <__main__.Pockle object at 0x0000000002EAE358>
protocol 2: __setstate__ is called!
<__main__.Pockle object at 0x0000000002EAE3C8>
protocol 3: __setstate__ is called!
<__main__.Pockle object at 0x0000000002EAE358>
So for protocols >= 2 setstate is called. This is caused by
object.__reduce_ex__ returning different tuples for different protocol versions:
>>> for p in range(4):
sys.stdout.write('protocol %d: %s\n' % (p, Pockle().__reduce_ex__(p)))
protocol 0: (<function _reconstructor at 0x0000000001F03048>, (<class
'__main__.Pockle'>, <class 'object'>, None))
protocol 1: (<function _reconstructor at 0x0000000001F03048>, (<class
'__main__.Pockle'>, <class 'object'>, None))
protocol 2: (<function __newobj__ at 0x0000000001F03148>, (<class
'__main__.Pockle'>,), 0, None, None)
protocol 3: (<function __newobj__ at 0x0000000001F03148>, (<class
'__main__.Pockle'>,), 0, None, None)
Implementation of reduce_ex for protos 0-1 in copy_reg.py contains the
documented check:
http://hg.python.org/cpython/file/f1509fc75435/Lib/copy_reg.py#l85
Implementation for proto 2+ in typeobject.c is happy with any value:
http://hg.python.org/cpython/file/f1509fc75435/Objects/typeobject.c#l3205
Pickle itself only ignores None, not any false value:
http://hg.python.org/cpython/file/f1509fc75435/Lib/pickle.py#l418
I think this is a documentation issue at this point.
[1] http://docs.python.org/py3k/library/pickle.html#pickle.object.__setstate__
----------
components: Library (Lib)
messages: 137935
nosy: eltoder
priority: normal
severity: normal
status: open
title: __setstate__ is called for false values
versions: Python 2.6, Python 2.7, Python 3.1, Python 3.2, Python 3.3
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue12290>
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