Strategies for backwards compatibility when using pickle?

[Chris Liechti]

"duncan" <duncan at enthought.com> wrote in 
news:mailman.219.1083624107.25742.python-list at python.org:

> I am working on an application that currently uses a simple pickle to
> save a project to disk. 
> 
> Every time we make a significant change to our object model we tell our
> beta users to delete all of their old projects. 
> 
> Does anyone have any advice on how to balance the conflicting interests
> of the developers who want to keep evolving the object model and users
> who need stable project persistance?

i sometimes use .ini files and the config parser o store program settings, 
but thats no option if you need to store entire object hierarchies.
the ini files are at least tolerant to new or removed values and easy to 
edit by hand, sometimes i pickle and hexlify objects to store them in the 
ini file too, but then i only do that with built in types to avoid such 
problems you mention above.

> It has been suggested that there may be a way to override pickle to deal
> with an evolving object model but it would be nice to hear from others
> who have tried to handle this problem. 

yep, especialy __getstate__ and __setstate__ are interesting for you. by 
implementing __setstate__ you can filter and update the pickled dictionary 
before you assign it to the new instance. thats a good way to convert old 
data to new objects.

http://python.org/doc/lib/pickle-inst.html

chris

-- 
Chris <cliechti at gmx.net>