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

[Tres Seaver]

On Fri, 24 Aug 2001, Thomas Olsen wrote:

> 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
> :-)

My mistake:  I said "spelling" when I meant to say "location of
the attribute assignment" (i.e., at class scope, rather than
bound to 'self');  the leading underscore *does* signal (by
convention in Python, and by check in Zope) "privateness" of the
attribute.

> > 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?

I could tell you, but then you'd have to scrape your brains off
of the ceiling. :)  Seriously, acquisition itself relies on
hooking '__getattr__' in fairly fiendish ways;  I am reluctant to
try any technique which might fight with it.

Tres.
-- 
===============================================================
Tres Seaver                                tseaver@zope.com
Zope Corporation      "Zope Dealers"       http://www.zope.com