pickling question

[Gary Robinson]

When you define a class in a script, and then pickle instances of that class in the same script and store them to disk, you can't load that pickle in another script. At least not the straightforward way [pickle.load(file('somefile.pickle'))]. If you try it, you get an AttributeError during the unpickling operation.

There is no problem, of course, if the class is defined in a module which is imported by the pickling script. pickle.load(file('somefile.pickle')) then works.

Rather than provide specific examples here, there's a blog post from 2005 that discusses this issue in depth and presents the problem very well: http://stefaanlippens.net/pickleproblem. (I tested in Python 2.6 yesterday and the same issue persists.)

Questions:

1) Does this have to be the case, or is it a design problem with pickles that should be remedied?

2) Is there an easier way around it than moving the class definition to a separate module? The blog post I point to above suggests putting "__module__ = os.path.splitext(os.path.basename(__file__))[0]" into the class definiton, but that's not working in my testing because when I do that, the pickling operation fails. Is there something else that can be done?

This is obviously not a huge problem. Substantial classes should usually be defined in a separate module anyway. But sometimes it makes sense for a script to define a really simple, small class to hold some data, and needing to create a separate module just to contain such a class can be a little annoying.

-- 

Gary Robinson
CTO
Emergent Music, LLC
personal email: garyrob at me.com
work email: grobinson at flyfi.com
Company: http://www.flyfi.com
Blog:    http://www.garyrobinson.net