Hi Phillip,

Is there a good reason that ExternalAttributeProvider sets up a 
dictionary to put its attributes into, rather than a PersistentMapping?

This is the way it seems to store things in the Rack:

Rack's BTree
   + object_id
      : PersistentMapping
        + ('ZPatterns.Rack', 'Self')
            : Persistent object, eg ZClass instance
        + ('ZPatterns.AttributeProviders','ExternalAttributes')
            : {}  # dict filled with external attributes

The thing is that when the dict gets updated by the External Attribute 
Provider, the whole second-level-in PersistentMapping object gets updated.

If an External Attribute Provider instead set up a new PersistentMapping 
in its slot, only that PersistentMapping would change in the ZODB on 

Do you think this is a reasonable change for the External Attribute 
Provider class? Or, should I write my own provider, derived from 
External Attribute Provider, that uses a PersistentMapping instead of a 

Actually, I can see the use for a Persistent AttributeProvider where you 
have a configurable slot, so you can group attributes that change at 
different rates in groups of those that change together.

The second half of the slot-key's tuple would be configurable as a 
property on the attribute provider.

Steve Alexander
Software Engineer
Cat-Box limited

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