Re: [Zope] Problem with properties in inherited ZClasses

[Steve Alexander]

Stephen Simmons wrote:

>
> 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

I tend to try not to have ZClasses derive from other ZClasses. You'd 
find it more straightforward either to derive both ZClasses from a 
common Python base class, or to keep both ZClasses independent of each 
other.

In general, I try to avoid using inheritence. Python and Zope support 
polymorphism whether you derive classes from the same base-class or not. 
Also, I find the coupling introduced by inheritence more trouble than it 
is worth for the reduced redundancy.

I realize that I have avoided trying to answer why ZClasses work in this 
way.

>   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'.

Here's the code that is raising the exception:

if hasattr(aq_base(self),id):
if not (id=='title' and not self.__dict__.has_key(id)):
raise 'Bad Request', (
'Invalid property id, %s. It is in use.' % id)

You can see that 'title' has a special status as a property.

 
> To conclude:
> 
> (i) What is going on and how can I fix it?

Don't do things that way to start with.

> (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?

If you add propertysheets to instances of your classes, rather than to 
the classes, then you don't get conflicts over attributes. However, 
unless you use ZPatterns or something like that, you have to refer to 
the attributes of propertysheets explicitly naming the propertysheet.

> (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.

Yes. See the code above from PropertySheets.py. Use something like this 
in an external method to find out the attributes of an object. (In 
python, methods and fields are both attributes.) You can use the magic 
aq_* attributes of acquisition wrappers to walk through the objects that 
get considered during acquisition. You might also want to look at Shane 
Hathaway's Acquisition Explainer, an external method to show how 
acquisition works, that also demonstrates use of the aq_* attributes.

--
Steve Alexander
Software Engineer
Cat-Box limited
http://www.cat-box.net





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[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
[EMAIL PROTECTED]


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