alphara commented on issue #7324:
URL: https://github.com/apache/pulsar/issues/7324#issuecomment-648523128


   The exception arises after the reader gets connected. Try to launch Pulsar 
before you launch the sample code.
   
   The pulsar docs says 
[here](https://pulsar.apache.org/docs/en/next/client-libraries-python/):
   > Pulsar Python client library is a wrapper over the existing C++ client 
library and exposes all of the same features. You can find the code in the 
python subdirectory of the C++ client code.
   Is this info obsolete?
   
   This is a simple example that I use
   ```python
   from pulsar import Client, MessageId
   
   client = Client(service_url='pulsar://localhost:6650',
                   authentication=None)
   topic = 'my-topic'
   msg_id = MessageId.earliest
   reader = client.create_reader(topic, msg_id)
   
   while True:
       msg = reader.read_next()
       print("Received message '{}' id='{}'".format(msg.data(), 
msg.message_id()))
   ```
   I found that the example above works on my machine.
   
   I have a bigger application that uses 
[asyncio](https://docs.python.org/3.7/library/asyncio.html) library. The 
`reader.read_next()` is executed inside the `run_in_executor()` method.
   ```python
     # ...
   
     async def _tryToSubscribe(self, topic, queue, callback, 
pulsarStartMessage):
       def sync_subscribe():
         reader = None
         startMessageId = MessageId.earliest
         reader = self.pulsar.create_reader(topic, startMessageId)     # <- 
exception occurs here
   
       await self.loop.run_in_executor(self.executor, sync_subscribe)
       # ...
   ```


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