I want my Python script to receive one message from a PubSub topic and then go on to other work. The code is learned from an example of the GCP document:

with subscriber:
    # The subscriber pulls a specific number of messages. The actual
    # number of messages pulled may be smaller than max_messages.
    response = subscriber.pull(
        request={"subscription": subscription_path, "max_messages": NUM_MESSAGES},
        retry=retry.Retry(deadline=300),
    )

    if len(response.received_messages) == 0:
        return

The problem is that it will receive empty messages, meaning that “len(response.received_messages)” is zero.

Where do these empty messages come from? Here is the answer:

Once a message is sent to a subscriber, the subscriber must either acknowledge or drop the message. A message is considered outstanding once it has been sent out for delivery and before a subscriber acknowledges it.

My solution is just to wait until receiving a non-empty message:

with subscriber:
    # The subscriber pulls a specific number of messages. The actual
    # number of messages pulled may be smaller than max_messages.
    while True:
      response = subscriber.pull(
          request={"subscription": subscription_path, "max_messages": NUM_MESSAGES},
          retry=retry.Retry(deadline=300),
      )

      if len(response.received_messages) > 0:
          break

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