Overriding properties

[Leif K-Brooks]

Nick Patavalis wrote:
> Why does the following print "0 0" instead of "0 1"? What is the
> canonical way to rewrite it in order to get what I obviously expect?
> 
>   class C(object):
<snip>
>       value = property(get_value, set_value)
>   
>   class CC(C):
>       def set_value(self, val):
<snip>
>   c.value = -1
>   cc.value = -1
>   print c.value, cc.value

The problem is that the names get_value and set_value are evaluated only 
once: when class C is first defined. The property has no knowledge of 
(or interest in) the fact that a different function named set_value has 
been defined.

There are several ways to fix it. The simplest would be to create a new 
property object in CC's definition:

class CC(C):
     def set_value(self, val):
         if val < 0:
             self.__val = 1
         else:
             self.__val = val
     value = property(C.get_value, set_value)

But an easier, more flexible method would be to use a metaclass which 
automatically adds properties for specially-named methods. One example 
of such a metaclass is autoprop from Guido van Rossum's descrintro 
essay; see <URL:http://www.python.org/2.2.3/descrintro.html>.