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from google.cloud import pubsub
subscriber = pubsub.SubscriberClient()

# Define the callback.
# Note that the callback is defined *before* the subscription is opened.
def callback(message):
    do_something_with(message)  # Replace this with your actual logic.
    message.ack()

# Open the subscription, passing the callback.
future = subscriber.subscribe(
    'projects/{project}/subscriptions/{subscription}',
    callback
)

# To block the thread you are in while messages are
# coming in the stream, use the result() method
try:
    future.result()
except Exception as ex:
    subscription.close()
    raise

# Finally, you can use cancel() to stop receiving messages.
future.cancel()