RE: [Zope] Subclassing Confusion

19 Jan 2000

      ...
-----Original Message-----
From: James W. Howe [mailto:jwh@allencreek.com]
Sent: Wednesday, January 19, 2000 1:20 PM
To: zope@zope.org
Subject: [Zope] Subclassing Confusion
I just tried to create a Python Base class for the purpose of 
overriding 
Images __str__ method and things didn't quite work as I would have 
expected.  I subclassed Image as follows (using the 
Renderable product as 
an example):
import Acquisition
from OFS import Image
You don't need these things...
...
class RenderableImage:
     def __str__ (self):
  Request = self.REQUEST
  if Request:
      try:
      Method = getattr (self, 'render')
      except AttributeError:
      return Image.__str__(self)
      return Method (self, Request)
  else:
      return "Renderable error: no request"
I defined a __init__ method to make the class a base class.
You don't need to define an __init__.
...
I then went 
into my Product and create a new ZClass which used 
RenderableImage as a 
base class.  Since I was just testing things, I immediately 
went and tried 
to create a new instance.  No problem creating the object.  
The problem (or 
my confusion) was the fact that my RenderableImage didn't 
inherit any of 
the views which an Image would normally have (i.e. Edit, Upload, 
Properties, View, Security).  My question is why didn't they 
show up?
Because you didn't actually subclass anything?
...
class RenderableImage:
                       ^^^^^
                       no declared subclasses....
     def __str__ (self):
...
I 
would have thought that absent any other changes to my Image 
subclass, that 
the default Image views would show up as the views of my new ZClass.
I think you're doing this a bit wrong, but that's understandable, it's
not documented anywhere.

Create a new Product in your lib/python/Products dir called
'MyImageProduct'.

In that Product, create a module called 'MyImageModule.py'

In that module, create

class MyImageClass:

  def __str__(self):
    return self.id         # whatever...

In the Product, create an __init__.py file:

import MyImageModule

def initialize(context):
  context.registerBaseClass(MyImageModule.MyImageClass)

Now, your ZClasses can subclass (aka, mix-in) the MyImageClass.  You
don't need to subclass Acquisition or Image at the Python level, just
create ZClass that subclasses both Image and  MyImageClass.  The ORDER
in which you subclass is important, because BOTH the Image and your
MyImageClass define an __str__ method, you want to subclass Image first,
then MyImageClass.

-Michel

RE: [Zope] Subclassing Confusion

Michel Pelletier