[Zope] Re: [Zope-CMF] Persistent dictionaries

[Thomas Olsen]

On Friday 24 August 2001 13:53, Tres Seaver wrote:

> >     _attributes = PersistentMapping()
>
> Note that this spelling make '_attributes' a "shared instance
> attribute" (all instances of the class, or ony class derived
> from it, will share the mapping.  

Oops - that certainly wasn't intentionally... I'm still very much of a Python 
newbie. Must have mixed it up with something else. I thought it was the 
notation for a pseudo private attribute. Need to pickup "Programming Python" 
a bit more often :-)

> Normally, I would expect to
> see the '_attibutes' assigned either in the initializer::
>
>        def __init__( self ):
>
>            self._attributes = PersistentMapping()
>
> or else "lazily"::
>
>        _attributes = None;
>
>        def __getattr__( self, name ):
>
>            if self._attributes is None:
>               self._attributes = PersistentMapping()
>
>            if self._attributes.has_key( name ): #...

The lazy approach looks very sensible.

> >
> > It works great - until the object gets unloaded from the memory :-(
> >
> > What am I doing wrong?
>
> Note as well that plaing with '__getattr__' in the presence of
> the acquisition machinery could be classifieds as Deep Voodoo (tm);

 :-) Are there any serious pitfalls I ought to know about?

> The canonical solution to your problem is to set a "shared" value
>
> for the new attribute on the class, with an appropriate default;
>
> instances which don't have the attribute in their own '__dict__'
> will find it in the class.  Where that solution is infeasible,
> the pickling machinery offers the '__setstate__' hook, which is
> called immediately on unpickling;  you would *really* prefer not
> to use this hook, but it is available.

Been there - done that ;-) But not really successfully.

> Tres.

-- 
Regards,
	Thomas Olsen

http://www.tanghus.dk