[Modeling-users] dynamic, metaclasses and Virtual properties

[Ernesto R. <er...@si...>]

Hi all,

I think that Python has a design error with properties, but it is easy 
to correct this. We can see that in the following example:

class Base(object):
    _name=None
    def getName(self):
        return self._name
    def setName(self, value):
        self._name=value
    name=property(getName,setName)

class Sub(Base):
    def setName(self, value):
       print "Setting name to '%s'" % value
       Base.setName(self,value)

Ok, here we can see how Sub extends the functionality of Base. But if we do:

instance=Sub()
instance.name="a name"

Sub.setName is NOT  called.!!!   Of course, it cand, because the 
property binds directly to the methods. Now we see the Base class 
corrected, using 'virtual' properties:

class Base(object):
    _name=None
    def getName(self):
        return self._name
    def setName(self, value):
        self._name=value
    name=property(lambda s: s.getName(), lambda s, v: s.setName(v))

class Sub(Base):
    def setName(self, value):
       print "Setting name to '%s'" % value
       Base.setName(self,value)

An try again:
 >>> instance=Sub()
 >>> instance.name="a name"
Setting name to 'a name'

In Modeling, when inheriting from class created with properties 
(dynamic), it does NOT the inherited methods. Is this reasonable? 

Here I have attached a little patch for dynamic.py. I made a local 
function mkProp, so that  variable binding is correct, otherwise  it 
does not work the way it should. Just a remark about the setter. As in 
the original code, a virtual setter is created only if the attribute is 
settable in the base class. In this case, we have two alternatives: 1. 
do not create a setter, as the original code, 2. create always a virtual 
setter and the setter in the base class raises a 
ReadOnlyAttributeException. (This is was I have done in a experimental 
framework.)

Best regards, Erny