title: Apache Beam Python SDK with Pub/Sub source stuck at runtime
body: <p>I am writing a program in Apache Beam using Python SDK to read from Pub/Sub the contents of a JSON file, and do some processing on the received string. This is the part in the program where I pull contents from Pub/Sub and do the processing:</p>
  
  <pre><code>with beam.Pipeline(options=PipelineOptions()) as pipeline:
      lines = pipeline | beam.io.gcp.pubsub.ReadStringsFromPubSub(subscription=known_args.subscription)
      lines_decoded = lines | beam.Map(lambda x: x.decode("base64"))
  
      lines_split = lines_decoded | (beam.FlatMap(lambda x: x.split('\n')))
  
      def json_to_tuple(jsonStr):
          res = json.loads(jsonStr)
          ##printing retutn value
          print (res['id'], res['messageSize'])
          ##
          return (res['id'], res['messageSize'])
  
      tupled = lines_split | beam.Map(json_to_tuple)
  
      def printlines(line):
          print line
  
      result = tupled | beam.CombinePerKey(sum)
      result | beam.Map(printlines)
  </code></pre>
  
  <p>While running the program, the code gets stuck after the creation of PCollection <code>tupled</code> (no lines of code is executing after that). The strange thing is that when I change the source from Pub/Sub to a local file which contain the <strong>exact same</strong> content (using <code>ReadFromText()</code>), the program is working perfectly.
  What could be the reason for this behaviour?</p>