Best way to add attributes to shelved objects?

[Jonathon McKitrick]

I'm working on an app that persists a few classes with shelve.  But if I
load older saved classes, I get an exception in code that uses the new
attributes, of course.

What is the best way to load these older classes, check for the new
attributes, and add them so they will work with the new code?

jm
-- 
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