Jan Eden wrote:
>  Hi,
>  
>  after updating a program to use new-style classes, I am a bit confused. My setup looks lik
e this (simplified):
>  
>  #Show.py
>  
>  import Data
>  
>  class Base(object):
>      ...
>      
>  class Page(Data.Page, Base):
>      ...
>  
>  class Author(Data.Author, Base):
>  ...
>  
>  #Data.py
>  
>  class Base:
>      ...
>  
>  class Page(Base):
>      ...
>  
>  class Author(Base):
>      ...
>  
>  The module Show.py provides methods, Data.py provides data
>  attributes. I use the property method in Data.Base for a certain
>  attribute - but why does this work?
>  
>  Data.Base has no base classes, so it is not based on a built-in type:
>  How can it be a new-style class?

Data.Base is not a new-style class, but the classes in Show are. And properties may appear t
o work in old-style classes. This document
http://www.python.org/2.2.3/descrintro.html#property
says,
Properties do not work for classic classes, but you don't get a clear error when you try thi
s. Your get method will be called, so it appears to work, but upon attribute assignment, a c
lassic class instance will simply set the value in its __dict__ without calling the property
's set method, and after that, the property's get method won't be called either.

Here is an old-style class that defines a property:

 > >> class OldStyle:
 ...     def getA(self):
 ...         print "getA"
 ...         return self.__a
 ...     def setA(self, value):
 ...         print "setA"
 ...         self.__a = value
 ...     a = property(getA, setA)
 ...
 > >> o=OldStyle()

OldStyle.getA() is called the first time I access o.a:
 > >> o.a
getA
Traceback (most recent call last):
  File "<stdin> ", line 1, in ?
  File "<stdin> ", line 4, in getA
AttributeError: OldStyle instance has no attribute '_OldStyle__a'

Setting a value for a wipes out the property, now it behaves like a normal attribute:
 > >> o.a = 3
 > >> o.a
3

A class that inherits from object and OldStyle is a new-style class and the property works a
s expected:
 > >> class NewStyle(object, OldStyle):
 ...   pass
 ...
 > >> n=NewStyle()
 > >> n.a
getA
Traceback (most recent call last):
  File "<stdin> ", line 1, in ?
  File "<stdin> ", line 4, in getA
AttributeError: 'NewStyle' object has no attribute '_OldStyle__a'
 > >> n.a=3
setA
 > >> n.a
getA
3

Kent

_______________________________________________
Tutor maillist  -  Tutor@[...].org
http://mail.python.org/mailman/listinfo/tutor