def add(self, chb):
        if chb.collection is not None:
            self.error(
                "Can't add checkbox as it already belongs "
                "to another collection!")
            return
        chb.collection = self
        self._items.append(chb)
ValidationError can be passed a string or a list.
dataset/ETHPy150Open tobspr/RenderPipeline/rpcore/gui/checkbox_collection.py/CheckboxCollection.add--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-0.96/django/core/validators.py/CriticalValidationError.__init__