[Zope] Semi-persistent objects, volatile attributes

[Jim Fulton]

Alexander Staubo wrote:
> 
> Is it possible to have persistent objects with attributes that are only
> persistent in memory?

Yes, but it takes some work and the mechanism need to be formalized.
Would you want to specify this on a per-class or per-instance basis?

> Effectively, an object that is always cached, for
> volatile attributes to be preserved during the Zope process' execution.
> Is there a way for an object to load itself permanently into the cache,
> say?

ditto
 
> I figure I could store the attributes in the product module as part of a
> dictionary keyed by the object instance, but this feels very dirty.

Right.
 
> Speaking of volatile attributes, is there an idiom for handling these?
> Since they're not stored, and __init__() is only ever called when the
> object is initially created (not persistently loaded), it seems you have
> to verify their existence, something like:
> 
>   def my_method(self):
>     self._v_foo = (hasattr(self, '_v_foo') and self._v_foo) or
> '<default>'

This is one way.
 
> ...ad tedium if you have lots of methods.

Yup.

> Does the persistence engine
> support an "onload" mechanism which you can override to know when your
> instance has been loaded and needs to initialize itself?

Yup, it's called __setstate__.  __setstate__ is called when
the object state is loaded. You can overload __setstate__
and initialize any volitile variables when you load your state:


class C:
  def __setstate__(self, state):
     C.inheritedAttribute('__setstate__')(self, state)
     self._v_spam=.....
 
Jim

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