'Derive property value.
Args:
model_instance: Instance to derive property for in bound method case,
else None.
model_class: Model class associated with this property descriptor.
Returns:
Result of calling self.__value_funcion as provided by property
constructor.'

def __get__(self, model_instance, model_class):

    if (model_instance is None):
        return self
    return self.__value_function(model_instance)
