[Zope] Problem with properties in inherited ZClasses

[Stephen Simmons]

Hi,

I have got stuck with inheritance and properties while writing a simple task
tracking product as a way to learn Zope (using a brand new 2.2.0 with no
other products). My product AdminTools currently has two ZClasses, Folder
and Task.

The ZClass AdminTools:Folder inherits from CatalogAware and ObjectManager.
Folder has a property sheet 'data', with properties 'title' (string) and
'description' (text). The Folder class has methods for displaying info about
the folder and the objects it contains.

I have tried to create another ZClass, AdminTools:Task that inherits from
AdminTools:Folder that adds some task tracking functionality and
uses/overrides the AdminTools:Folder methods for displaying objects. Task
also has a property sheet named 'data' with properties 'title' (string), due
date (date) and owner (string). I tried to add a final property
'description' (text) to AdminTools:Task, but get this error saying it is
already in use:

Invalid property id, description. It is in use.
Traceback (innermost last):
  File D:\Dev\ZopeTestbed\lib\python\ZPublisher\Publish.py, line 222, in
publish_module
  File D:\Dev\ZopeTestbed\lib\python\ZPublisher\Publish.py, line 187, in
publish
  File D:\Dev\ZopeTestbed\lib\python\Zope\__init__.py, line 221, in
zpublisher_exception_hook
    (Object: Traversable)
  File D:\Dev\ZopeTestbed\lib\python\ZPublisher\Publish.py, line 171, in
publish
  File D:\Dev\ZopeTestbed\lib\python\ZPublisher\mapply.py, line 160, in
mapply
    (Object: manage_addProperty)
  File D:\Dev\ZopeTestbed\lib\python\ZPublisher\Publish.py, line 112, in
call_object
    (Object: manage_addProperty)
  File D:\Dev\ZopeTestbed\lib\python\OFS\PropertySheets.py, line 427, in
manage_addProperty
    (Object: Traversable)
  File D:\Dev\ZopeTestbed\lib\python\OFS\PropertySheets.py, line 235, in
_setProperty
    (Object: Task)
Bad Request: (see above)

I don't understand why I can add the 'title' property to Task but not
'description'. Both 'title' and 'description' are defined in the inherited
class Folder, so they are being treated differently for some reason. A
little while ago, I had the same error while trying to create a property
named 'summary'. At the time, I thought it was because of a clash with the
summary() method of CatalogAwareness. But now I'm confused.

To conclude:

(i) What is going on and how can I fix it?

(ii) When using a ZClass as a base class, do instances inherit any
properties, or do they just get access to the base class's methods? If
ZClasses do inherit properties from parent ZClasses, how do I reach them via
the property sheets?

(iii) Is there a simple way to find out what methods and properties an
object has? I would like to see what it defines, what it inherits and what
it acquires.

_______________________________
Stephen Simmons
HealthArena B.V.
phone +31 20 486 0555
mobile +31 6 1505 3086
stephen.simmons@healtharena.net