class attribute to instance attribute

Donnal Walter donnal at donnal.net
Thu Jun 30 22:50:26 CEST 2005


This is a question about Python patterns or idioms. Over a period of 
time, I have evolved a pattern of usage that seems to work better for me 
than other ways I tried previously, but in writing some documentation I 
don't know what to call this syntax or how best to describe it. I have 
not seen it used in other places.

The somewhat longish version is that I have implemented an MVP 
(model-view-presenter) architecture where each "presenter" has a "view" 
as an attribute. Take the TextField and Frame presenters for example. 
Here is one way to do it:

import wx
class TextField:

     def __init__(self):
         # do some things here to calculate *args
         self.view = wx.TextCtrl(*args)
         # do more things to set up the view

     # and more methods to make this a presenter


class Frame:

     def __init__(self):
         # do some things here to calculate *args
         self.view = wx.Frame(*args)
         # do more things to set up the view

     # and more methods to make this a presenter


There are a number of presenters, some of which have the same type of 
view (TextField and NumberField, for example) and many of which have 
different views (Frame and Notebook, for example). The way I have chosen 
to do this is to put the logic in one superclass, "Presenter":

class Presenter:
     view = None

     def __init__(self):
         # do some things here to calculate *args
         # if view is class, create instance
         if callable(self.view):
             self.view = self.view(*args)
         # do more things to set up the view

     # and more methods to make this a presenter


class TextField(Presenter):
     view = wx.TextCtrl

class Frame(Presenter):
     view = wx.Frame


Then:
 >>> app = wx.App(False)
 >>> f = Frame()
 >>> isinstance(f.view, wx.Frame)
True

To summarize, each subclass has a class attribute "view" that is 
converted to an instance attribute of the same name at runtime.

Is this a common Python idiom? If so, does it have a name? Is there a 
better way to do the same thing?

Regards,
Donnal Walter
Arkansas Children's Hospital





More information about the Python-list mailing list