[Zope3-dev] class lifecycle management/missing attributes

[Guido van Rossum]

> Is there an example in the Zope 3 code that illustrates a solution to
> the following problem:
> 
> 1 - A component of class Foo is created in the ZODB.
> 
> 2 - Class Foo is modified, adding a new attribute in __init__:
> 
>     def __init__(self):
>       self.bar = 'some_default'
> 
> 3 - The component in the ZODB doesn't have bar -- bad.
> 
> How would one add bar to instances of Foo already serialized? In other
> apps, I've handled this by adding the missing attribute in a deserialize
> method.
> 
> I assume there's a pattern for this, but haven't spotted anything in the
> source.

If all you want is a default with an immutable value, you can put it
in the class instead of in __init__.

Otherwise, overriding __setstate__ would be an option, but I don't
know if it's the best practice.

--Guido van Rossum (home page: http://www.python.org/~guido/)