[Zope-dev] Persistence and __getattr__

[Jim Fulton]

Kent Polk wrote:
> 
> It appears to me that if you have a class object that you want to
> be Persistence, all the object attributes have to be in object.__dict__
> or Persistence.__getattr__ has no way of retrieving them.

This is right, unless you are willing to go down to the C level.
 
> This at first seems pretty obvious, but what about instances where
> your object attributes change outside of zope?

Why are these objects persistent?  If they are persistent, then 
their data is stored in Zope and doesn't change outside of
Zope.

> I can't determine
> any way to trigger an 'attribute refresh' on a __getattr__ failure
> if the attribute does not exist in the object.__dict__.

Right.
 
> Is this correct?

Yes.  It sounds like you don't want a persistent object at all.
You want an object that get's it's data from somewhere else
(somewhere other than the object database).

You can have a sub-object of a persistent object that is not persistent
and gets *it's* data from somewhere else.  This is essentially what
happens with database connections and SQL methods. 

> Is it that 'volatile' attributes cannot be Persistent?

There's no point in making them persistent, since they won't be
stored in the database. :)
 
> I see that when I try to override Persistence.__getattr__ in order
> to add the capability to create or change attributes while still
> seemingly duplicating the the method instructions, I encounter an
> infinite loop. :^)  Yes, I read the docs, but I'm trying to figure
> out if there is perhaps a way around this puzzle.

Don't override __getattr__ or __setattr__ for subclasses of Persistent.
(It *can* be done, but it requires such *deep* zen that it makes *my*
head hurt. There is actually a class used by ZClasses that does this. 
:)

Jim

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