Serialize obj into JSON.

def default(self, obj):
        """Serialize obj into JSON."""
        # pylint: disable=method-hidden, protected-access, arguments-differ
        if isinstance(obj, Sensor):
            return {
                'sensor_id': obj.sensor_id,
                'children': obj.children,
                'type': obj.type,
                'sketch_name': obj.sketch_name,
                'sketch_version': obj.sketch_version,
                'battery_level': obj.battery_level,
                'protocol_version': obj.protocol_version,
                'heartbeat': obj.heartbeat,
            }
        if isinstance(obj, ChildSensor):
            return {
                'id': obj.id,
                'type': obj.type,
                'description': obj.description,
                'values': obj.values,
            }
        return json.JSONEncoder.default(self, obj)