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serialization and versioning
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Neal Becker
Oct 12, 2012, 6:42:03 AM10/12/12
to pytho...@python.org
I wonder if there is a recommended approach to handle this issue.
Suppose objects of a class C are serialized using python standard pickling.
Later, suppose class C is changed, perhaps by adding a data member and a new
constructor argument.
It would see the pickling protocol does not directly provide for this - but is
there a recommended method?
I could imagine that a class could include a class __version__ property that
might be useful - although I would further expect that it would not have been
defined in the original version of class C (but only as an afterthought when it
became necessary).
Suppose objects of a class C are serialized using python standard pickling.
Later, suppose class C is changed, perhaps by adding a data member and a new
constructor argument.
It would see the pickling protocol does not directly provide for this - but is
there a recommended method?
I could imagine that a class could include a class __version__ property that
might be useful - although I would further expect that it would not have been
defined in the original version of class C (but only as an afterthought when it
became necessary).
Etienne Robillard
Oct 12, 2012, 7:04:37 AM10/12/12
to Neal Becker, pytho...@python.org
> http://mail.python.org/mailman/listinfo/python-list
i guess a easy answer is to say to try python 3.3 but how would this translate in
python (2) code ?
Neal Becker
Oct 12, 2012, 8:50:56 AM10/12/12
to pytho...@python.org
Robert Kern
Oct 12, 2012, 9:26:19 AM10/12/12
to pytho...@python.org
On 10/12/12 11:42 AM, Neal Becker wrote:
> I wonder if there is a recommended approach to handle this issue.
>
> Suppose objects of a class C are serialized using python standard pickling.
> Later, suppose class C is changed, perhaps by adding a data member and a new
> constructor argument.
>
> It would see the pickling protocol does not directly provide for this - but is
> there a recommended method?
>
> I could imagine that a class could include a class __version__ property that
> might be useful - although I would further expect that it would not have been
> defined in the original version of class C (but only as an afterthought when it
> became necessary).
You don't need to add anything to the actual attributes on the class. Just
> I wonder if there is a recommended approach to handle this issue.
>
> Suppose objects of a class C are serialized using python standard pickling.
> Later, suppose class C is changed, perhaps by adding a data member and a new
> constructor argument.
>
> It would see the pickling protocol does not directly provide for this - but is
> there a recommended method?
>
> I could imagine that a class could include a class __version__ property that
> might be useful - although I would further expect that it would not have been
> defined in the original version of class C (but only as an afterthought when it
> became necessary).
define your __getstate__() dict to include a '__version__' item. You can treat
the absence of '__version__' in the dict that __setstate__() receives as
implying "version 0".
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
Dieter Maurer
Oct 13, 2012, 2:23:14 AM10/13/12
to pytho...@python.org
Neal Becker <ndbe...@gmail.com> writes:
> I wonder if there is a recommended approach to handle this issue.
>
> Suppose objects of a class C are serialized using python standard pickling.
> Later, suppose class C is changed, perhaps by adding a data member and a new
> constructor argument.
>
> It would see the pickling protocol does not directly provide for this - but is
> there a recommended method?
>
> I could imagine that a class could include a class __version__ property that
> might be useful - although I would further expect that it would not have been
> defined in the original version of class C (but only as an afterthought when it
> became necessary).
The ZODB (Zope Object DataBase) is based on Python's pickle.
> I wonder if there is a recommended approach to handle this issue.
>
> Suppose objects of a class C are serialized using python standard pickling.
> Later, suppose class C is changed, perhaps by adding a data member and a new
> constructor argument.
>
> It would see the pickling protocol does not directly provide for this - but is
> there a recommended method?
>
> I could imagine that a class could include a class __version__ property that
> might be useful - although I would further expect that it would not have been
> defined in the original version of class C (but only as an afterthought when it
> became necessary).
In the ZODB world, the following strategy is used:
* if the class adds a new data attribute, give it (in addition) a
corresponding class level attribute acting as "default" value
in case the pickled state of an instance lacks this
instance level attribute
* for more difficult cases, define an appropriate "__getstate__"
for the class that handles the necessary model upgrades
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