[ZODB-Dev] Persistent object has empty __dict__ for a little while

[Jim Fulton]

On Wed, Jan 6, 2010 at 10:40 AM, Martin Aspeli <optilude+lists at gmail.com> wrote:
> Hi,
>
> This one is pretty high no the list of weirdest things to have happened
> to me in a while. Basically, I have a persistent object that has an
> empty __dict__() on the first request, until it suddenly decides to have
> data again.
>
> I'm on ZODB 3.9.3, using Zope 2.12 and Plone 4. I have a persistent
> object, /plone/portal_registry:
>
>   class Registry(registry.Registry, SimpleItem):
>       pass
>
> The base class is:
>
>   class Registry(Persistent):
>       def __init__(self):
>           self._records = Records(self)
>
> Records is:
>
>     class Records(Persistent):
>
>       __parent__ = None
>
>       def __init__(self, parent):
>           self.__parent__ = parent
>           self.data = OOBTree()
>
>       def __getitem__(self, name):
>           return self.data.__getitem__(name)
>
> The BTree contains string keys and Record objects as values. Record is:
>
>     class Record(Persistent):
>         __name__ = u""
>         __parent__ = None
>
>         field = None
>
> __parent__ is set to records.__parent__, which is going to be the
> Registry object.
>
> field is set to a special persistent field class inheriting from
> zope.schema's fields, but mixing in persistence. In this case, I have a
> list field:
>
>   class PersistentField(persistent.Persistent):
>       """Base class for persistent field definitions.
>       """
>
>   class PersistentCollectionField(PersistentField,
>              zope.schema._field.AbstractCollection):
>
>   class List(PersistentCollectionField, zope.schema.List):
>       pass
>
> I then have some code like this:
>
>     def cloneField(self, field):
>         clone = field.__class__.__new__(field.__class__)
>         clone.__dict__.update(field.__dict__)
>         for name, attr in field.__dict__.items():
>             if IField.providedBy(attr):
>                 clone.__dict__[name] = self.cloneField(attr)
>         return clone
>
> This is used like so:
>
>     registry = getUtility(IRegistry) # persistent/local
>     clone = self.cloneField(registry.records[recordName].field)
>
> This seems to work always *except* on the first request immediately
> after starting up Zope. In this case, field.__dict__ is {}. If I poke at
> it long enough in pdb, it suddenly springs into life and contains values
> again.
>
> Can anyone enlighten me as to:
>
>  - why this may happen
>  - how I can ensure it stops happening :-)

__dict__ is a special variable. Accessing it doesn't cause an object
to be activated.

Rather than doing:

   clone.__dict__.update(field.__dict__)

I would do:

   clone.__setstate__(field.__getstate__)

Alternatively, you can assure that field is acticated first, using
_p_activate or by accessing an
attribute.

Jim

-- 
Jim Fulton