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Date: Mon, 2 Apr 2012 18:48:45 -0400
Subject: Re: why can't I pickle a class containing this dispatch dictionary?
From: Michael Hrivnak <mhrivnak@hrivnak.org>
To: jkn <jkn_gg@nicorp.f9.co.uk>
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Xref: csiph.com comp.lang.python:22553

Pickle cannot pickle a reference to an instance method.  So the
problem is that self.myDict has values which are references to
instance methods.

Without questioning what this is trying to do or why (I assume it's a
proof of concept), here is a way to make it picklable:
http://pastebin.com/1zqE52mD

Michael

On Mon, Apr 2, 2012 at 7:17 AM, jkn <jkn_gg@nicorp.f9.co.uk> wrote:
> Hi All
> =A0 =A0I'm clearly not understanding the 'can't pickle instancemethod
> objects' error; can someone help me to understand, & maybe suggest a
> workaround, (apart from the obvious if ... elif...).
>
> I'm running Python 2.6 on an embedded system.
>
> =3D=3D testpickle.py =3D=3D
> import pickle
>
> class Test(object):
> =A0 =A0def __init__(self):
> =A0 =A0 =A0 =A0self.myDict =3D {
> =A0 =A0 =A0 =A0 =A0 =A01: self.tag1,
> =A0 =A0 =A0 =A0 =A0 =A02: self.tag2
> =A0 =A0 =A0 =A0 =A0 =A0}
> =A0 =A0def dispatch(self, v):
> =A0 =A0 =A0 =A0try:
> =A0 =A0 =A0 =A0 =A0 =A0self.myDict[v]()
> =A0 =A0 =A0 =A0except KeyError:
> =A0 =A0 =A0 =A0 =A0 =A0print "No corresponding dictionary entry!"
> =A0 =A0 =A0 =A0#
> =A0 =A0def tag1(self):
> =A0 =A0 =A0 =A0print "one"
> =A0 =A0def tag2(self):
> =A0 =A0 =A0 =A0print "two"
>
>
> t =3D Test()
> t.dispatch(1)
> t.dispatch(2)
> t.dispatch(0)
>
> fd =3D open("pickle.out", "w")
> pickle.dump(t, fd)
> fd.close()
> # EOF
>
> $ python testpickle.py
> one
> two
> No corresponding dictionary entry!
> Traceback (most recent call last):
> =A0File "ptest.py", line 29, in <module>
> =A0 =A0pickle.dump(t, fd)
> =A0File "/usr/lib/python2.6/pickle.py", line 1362, in dump
> =A0 =A0Pickler(file, protocol).dump(obj)
> =A0File "/usr/lib/python2.6/pickle.py", line 224, in dump
> =A0 =A0self.save(obj)
> =A0File "/usr/lib/python2.6/pickle.py", line 331, in save
> =A0 =A0self.save_reduce(obj=3Dobj, *rv)
> =A0File "/usr/lib/python2.6/pickle.py", line 419, in save_reduce
> =A0 =A0save(state)
> =A0File "/usr/lib/python2.6/pickle.py", line 286, in save
> =A0 =A0f(self, obj) # Call unbound method with explicit self
> =A0File "/usr/lib/python2.6/pickle.py", line 649, in save_dict
> =A0 =A0self._batch_setitems(obj.iteritems())
> =A0File "/usr/lib/python2.6/pickle.py", line 663, in _batch_setitems
> =A0 =A0save(v)
> =A0File "/usr/lib/python2.6/pickle.py", line 286, in save
> =A0 =A0f(self, obj) # Call unbound method with explicit self
> =A0File "/usr/lib/python2.6/pickle.py", line 649, in save_dict
> =A0 =A0self._batch_setitems(obj.iteritems())
> =A0File "/usr/lib/python2.6/pickle.py", line 663, in _batch_setitems
> =A0 =A0save(v)
> =A0File "/usr/lib/python2.6/pickle.py", line 306, in save
> =A0 =A0rv =3D reduce(self.proto)
> =A0File "/usr/lib/python2.6/copy_reg.py", line 70, in _reduce_ex
> =A0 =A0raise TypeError, "can't pickle %s objects" % base.__name__
> TypeError: can't pickle instancemethod objects
> $
>
>
> =A0 =A0Thanks
> =A0 =A0J^n
>
> --
> http://mail.python.org/mailman/listinfo/python-list