Re: 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>.
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