class Descr(object):
    def __get__(self, obj, objtype):
        if obj is None:
            value =  objtype.__dict__['_foo']
            print "__get__ (on class): ", value
        else:
            value =  obj.__dict__['_foo']
            print "__get__: ", value
        return value

    def __set__(self, obj, value, objtype=None):
        if obj is None:
            print "__set__ (on class): ", value
            objtype.__dict__['_foo'] = value
        else:
            print "__set__: ", value
            obj.__dict__['_foo'] = value

class CoolerSetAttr(type):
  	def __setattr__(cls, name, value):
   	  	if name in cls.__dict__ and hasattr(cls.__dict__[name], '__set__'):
   	  	  	cls.__dict__[name].__set__(None, value, cls)
   	  	else:
   	  	  	type.__setattr__(name, value)

class Foo(object):
    __metaclass__ = CoolerSetAttr
    
    _foo = 90
    foo = Descr()

    def __init__(self):
        self._foo = 123

f = Foo()
print f.foo

f.foo = 456
print f.foo

print "========================"

print Foo.foo

Foo.foo = 456
print Foo.foo