Return copy of consumer subscription set (#1029)

kafka/consumer/group.py

@@ -819,7 +819,7 @@ class KafkaConsumer(six.Iterator):
         Returns:
             set: {topic, ...}
         """
-        return self._subscription.subscription
+        return self._subscription.subscription.copy()
 
     def unsubscribe(self):
         """Unsubscribe from all topics and clear all assigned partitions."""

test/test_consumer.py

@@ -24,6 +24,14 @@ class TestKafkaConsumer(unittest.TestCase):
         with self.assertRaises(KafkaConfigurationError):
             KafkaConsumer(bootstrap_servers='localhost:9092', fetch_max_wait_ms=41000, request_timeout_ms=40000)
 
+    def test_subscription_copy(self):
+        consumer = KafkaConsumer('foo', api_version=(0, 10))
+        sub = consumer.subscription()
+        assert sub is not consumer.subscription()
+        assert sub == set(['foo'])
+        sub.add('fizz')
+        assert consumer.subscription() == set(['foo'])
+
 
 class TestMultiProcessConsumer(unittest.TestCase):
     @unittest.skipIf(sys.platform.startswith('win'), 'test mocking fails on windows')