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Why derivated exception can not be pickled ?

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  Why derivated exception can not be pickled ? Mathieu Courtois <mathieu.courtois@gmail.com> - 2012-09-04 08:57 -0700
    Re: Why derivated exception can not be pickled ? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-05 00:34 +0000
    Re: Why derivated exception can not be pickled ? Dieter Maurer <dieter@handshake.de> - 2012-09-05 08:02 +0200
      Re: Why derivated exception can not be pickled ? Mathieu Courtois <mathieu.courtois@gmail.com> - 2012-09-05 01:54 -0700
      Re: Why derivated exception can not be pickled ? Mathieu Courtois <mathieu.courtois@gmail.com> - 2012-09-05 01:54 -0700
    Re: Why derivated exception can not be pickled ? Mathieu Courtois <mathieu.courtois@gmail.com> - 2012-09-05 03:18 -0700

#28404 — Why derivated exception can not be pickled ?

Here is my example :


import cPickle

ParentClass = object     # works
ParentClass = Exception  # does not

class MyError(ParentClass):
    def __init__(self, arg):
        self.arg = arg

    def __getstate__(self):
        print '#DBG pass in getstate'
        odict = self.__dict__.copy()
        return odict

    def __setstate__(self, state):
        print '#DBG pass in setstate'
        self.__dict__.update(state)

exc = MyError('IDMESS')

fo = open('pick.1', 'w')
cPickle.dump(exc, fo)
fo.close()

fo = open('pick.1', 'r')
obj = cPickle.load(fo)
fo.close()


1. With ParentClass=object, it works as expected.

2. With ParentClass=Exception, __getstate__/__setstate__ are not called.

Does anyone explain me why ?
Thanks.

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#28438

On Tue, 04 Sep 2012 08:57:00 -0700, Mathieu Courtois wrote:

> Here is my example :
> 
> 
> import cPickle
> 
> ParentClass = object     # works
> ParentClass = Exception  # does not
[...]
> 1. With ParentClass=object, it works as expected.
> 
> 2. With ParentClass=Exception, __getstate__/__setstate__ are not called.
> 
> Does anyone explain me why ?


I think it is a bug. According to the documentation, if your class is 
unpickleable, an exception should be raised. If it is pickleable, 
__getstate__ should be called. I can't see anything to explain that what 
you are seeing is expected behaviour.


Exceptions should definitely be pickleable:

http://bugs.python.org/issue1692335

so __getstate__ should be called. I think you should report this as a bug.


-- 
Steven

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#28454

Mathieu Courtois <mathieu.courtois@gmail.com> writes:

> Here is my example :
>
>
> import cPickle
>
> ParentClass = object     # works
> ParentClass = Exception  # does not
>
> class MyError(ParentClass):
>     def __init__(self, arg):
>         self.arg = arg
>
>     def __getstate__(self):
>         print '#DBG pass in getstate'
>         odict = self.__dict__.copy()
>         return odict
>
>     def __setstate__(self, state):
>         print '#DBG pass in setstate'
>         self.__dict__.update(state)
>
> exc = MyError('IDMESS')
>
> fo = open('pick.1', 'w')
> cPickle.dump(exc, fo)
> fo.close()
>
> fo = open('pick.1', 'r')
> obj = cPickle.load(fo)
> fo.close()
>
>
> 1. With ParentClass=object, it works as expected.
>
> 2. With ParentClass=Exception, __getstate__/__setstate__ are not called.

The pickle interface is actually more complex and there are several
ways an object can ensure picklability. For example, there is
also a "__reduce__" method. I suppose, that "Exception" defines methods
which trigger the use of an alternative picklability approach (different
from "__getstate__/__setstate__").

I would approach your case the following way: Use "pickle" instead
of "cPickle" and debug picking/unpickling to find out what
happens in detail.

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#28468

Thanks for your reply


On Wednesday, September 5, 2012 8:02:55 AM UTC+2, Dieter Maurer wrote:
> 
> The pickle interface is actually more complex and there are several
> 
> ways an object can ensure picklability. For example, there is
> 
> also a "__reduce__" method. I suppose, that "Exception" defines methods
> 
> which trigger the use of an alternative picklability approach (different
> 
> from "__getstate__/__setstate__").

You're right: Exception has __reduce__ & __reduce_ex__ methods. Always read carefully the manual ;-)

I must override these methods.


MC

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#28469

Thanks for your reply


On Wednesday, September 5, 2012 8:02:55 AM UTC+2, Dieter Maurer wrote:
> 
> The pickle interface is actually more complex and there are several
> 
> ways an object can ensure picklability. For example, there is
> 
> also a "__reduce__" method. I suppose, that "Exception" defines methods
> 
> which trigger the use of an alternative picklability approach (different
> 
> from "__getstate__/__setstate__").

You're right: Exception has __reduce__ & __reduce_ex__ methods. Always read carefully the manual ;-)

I must override these methods.


MC

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#28476

Hello,

The simple example works fine using __reduce__:

class MyError(Exception):
    def __init__(self, arg):
        self.arg = arg

    def __reduce__(self):
        return (MyError, (self.arg, ))

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