FastAPI endpoint blocked until a message has been consumed with aio-pika

Question

I am using a FastAPI endpoint to do some sort of long polling and also aiopika to wait messages that a client is publishing to rabbitmq connection/channel/exchange/queue.

My question: When I make a request to the endpoint it is blocked waiting for a message to arrive but as you can see in the below snipped if I put a print() instruction before it starts consuming it is not shown until a message arrives. Why this happens?

@router.post("/long_polling")
async def long_polling(request):
   device_id = "1234"

   print("THIS IS NOT PRINTED UNTIL A MESSAGE IS PUBLISHED!!!")

   connection = await aio_pika.connect("amqp://user:pwd@rabbitmq:5672/")
   async with connection:
        channel = await connection.channel()
        exchange = await channel.declare_exchange("exchange", aio_pika.ExchangeType.TOPIC)
        queue = await channel.declare_queue("")
        await queue.bind(exchange, routing_key="{}.outgoing".format(device_id))

        print("NOT PRINTED UNTIL A MESSAGE ARRIVE ALTHOUGH BEFORE CONSUMING")

        # START CONSUMING
        async with queue.iterator() as queue_iter:
            async for message in queue_iter:
                async with message.process():
                    print(message.body)
                    return

When I published a message, the endpoint returns properly, so just to know why the previous instructions are not executed until a new message arrives.