'Weigh multiple objects.
Override in a subclass if you need access to all objects in order
to calculate weights. Do not modify the weight of an object here,
just return a list of weights.'

def weigh_objects(self, weighed_obj_list, weight_properties):

    weights = []
    for obj in weighed_obj_list:
        weight = self._weigh_object(obj.obj, weight_properties)
        if (self.minval is None):
            self.minval = weight
        if (self.maxval is None):
            self.maxval = weight
        if (weight < self.minval):
            self.minval = weight
        elif (weight > self.maxval):
            self.maxval = weight
        weights.append(weight)
    return weights
