issue-51-telegram fix bug when no mbus devices are present

dsmr_parser/objects.py

@@ -36,9 +36,12 @@ class Telegram(object):
         # Group Mbus related values into a MbusDevice object.
         # TODO MaxDemandParser (BELGIUM_MAXIMUM_DEMAND_13_MONTHS) returns a list
         if isinstance(dsmr_object, DSMRObject) and dsmr_object.is_mbus_reading:
-            channel_id = dsmr_object.obis_id_code[1]
-            mbus_device = self._mbus_devices[channel_id]
-            mbus_device.add(obis_reference, dsmr_object)
+            self._add_mbus(obis_reference, dsmr_object)
+
+    def _add_mbus(self, obis_reference, dsmr_object):
+        channel_id = dsmr_object.obis_id_code[1]
+        mbus_device = self._mbus_devices[channel_id]
+        mbus_device.add(obis_reference, dsmr_object)
 
     def get_mbus_devices(self):
         """
@@ -47,7 +50,9 @@ class Telegram(object):
         return [d[1] for d in sorted(self._mbus_devices.items(), key=lambda x: x[0])]
 
     def get_mbus_device_by_channel(self, channel_id):
-        return self._mbus_devices[channel_id]
+        # Check key, because defaultdict would otherwise instantiate an empty MbusDevice
+        if channel_id in self._mbus_devices:
+            return self._mbus_devices[channel_id]
 
     def __getattr__(self, name):
         """ will only get called for undefined attributes """

test/test_telegram.py

@@ -356,3 +356,14 @@ class TelegramTest(unittest.TestCase):
         self.assertEqual(mbus_device_2.DEVICE_TYPE.value, 3)
         self.assertEqual(mbus_device_2.EQUIPMENT_IDENTIFIER_GAS.value, '4730303339303031393336393930363139')
         self.assertEqual(mbus_device_2.HOURLY_GAS_METER_READING.value, Decimal('246.138'))
+
+    def test_without_mbus_devices(self):
+        parser = TelegramParser(telegram_specifications.V5, apply_checksum_validation=False)
+        telegram = parser.parse('')
+
+        self.assertEqual(telegram.get_mbus_devices(), [])
+        self.assertIsNone(telegram.get_mbus_device_by_channel(1))
+
+        # Because of a bug related to incorrect use of defaultdict,
+        # test again for unwanted side effects
+        self.assertEqual(telegram.get_mbus_devices(), [])