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):
        print "__set__: ", value
        obj.__dict__['_foo'] = value

class Foo(object):

    _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