Pickling/Unpickling python Exceptions

[Sergey Lukin]

Is anybody there to help me out ?

On Thu, Jan 27, 2011 at 4:49 PM, Sergey Lukin <lukin.s.v at gmail.com> wrote:

> Hi all,
>
> I'm migrating code from python 2.4 to python 2.6 and I've got into troubles
> with pickling/unpickling  python Exceptions.
> The following code works fine in 2.4 but not in 2.6.
> See Exception1 example
>
> I have found on python mail list similar problem
> http://mail.python.org/pipermail/python-list/2009-December/1228773.html
> They recommend to use __reduce__. But this does not help as I'm getting
> different Exception class after pickle
> See Exception_with_reduce example
>
> I also have found possible solution to this problem here
> http://bugs.python.org/issue1692335
> As a workaround they propose to pass Exception arguments into base class
> See Exception2 example
> But there is another problem. Constructor is called 2 times which is not
> acceptable to me.
>
> Could you please advice on the solution?
>
> ------------------------------------------
> test program
> ------------------------------------------
> import cPickle
>
> class Exception1(Exception):
>     def __init__(self,arg1):
>         print "constructor called"
>         Exception.__init__(self)
>
> class Exception2(Exception):
>     def __init__(self,arg1):
>         print "constructor called"
>         Exception.__init__(self,arg1)
>
> class Exception_with_reduce(Exception):
>     def __reduce__(self):
>         try:
>             getnewargs = self.__getnewargs__
>         except AttributeError:
>             newargs = (self.__class__,)
>         else:
>             newargs = (self.__class__,) + getnewargs()
>
>         try:
>             getstate = self.__getstate__
>         except AttributeError:
>             state = self.__dict__
>         else:
>             state = getstate()
>         return (Exception, newargs, state)
>
>     def __init__(self,arg1):
>         print "constructor called"
>         Exception.__init__(self,arg1)
>
> def test(E,args):
>     try:
>         print ">>",E.__name__
>         e = E(*args)
>         print "- pickling"
>         s = cPickle.dumps(e)
>         print "- unpickling"
>         e = cPickle.loads(s)
>
>         if E != e.__class__:
>             print "! failed: expected %s, got
> %s"%(E.__name__,e.__class__.__name__)
>     except Exception, e:
>         print "! failed:",e
>
>     print "\ finished"
>     print
>
>
> import os
> if os.path.isfile("/home/ast1/blabla"):
>     try:
>         s = open("/home/ast1/blabla","r").read()
>         e = cPickle.loads(s)
>         print e.__class__
>     except Exception, e:
>         print "error:",e
>
> test(Exception1,[1])
> test(Exception2,[1])
> test(Exception_with_reduce,[1])
> ------------------------------------------
>
>
> ------------------------------------------
> run results on python 2.6:
> ------------------------------------------
>
> constructor called
> <class '__main__.Exception2'>
> >> Exception1
> constructor called
> - pickling
> - unpickling
> ! failed: ('__init__() takes exactly 2 arguments (1 given)', <class
> '__main__.Exception1'>, ())
> \ finished
>
> >> Exception2
> constructor called
> - pickling
> - unpickling
> constructor called
> \ finished
>
> >> Exception_with_reduce
> constructor called
> - pickling
> - unpickling
> ! failed: expected Exception_with_reduce, got Exception
> \ finished
>
> ------------------------------------------
> run results on python 2.4:
> ------------------------------------------
>
> __main__.Exception2
> >> Exception1
> constructor called
> - pickling
> - unpickling
> \ finished
>
> >> Exception2
> constructor called
> - pickling
> - unpickling
> \ finished
>
> >> Exception_with_reduce
> constructor called
> - pickling
> - unpickling
> \ finished
>
>
>
>
>


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