[GitHub] [pulsar] candlerb opened a new issue #5518: python functions cannot find python3 interpreter

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candlerb opened a new issue #5518: python functions cannot find python3 interpreter
URL: https://github.com/apache/pulsar/issues/5518
 
 
   **Describe the bug**
   python functions do not start if the system has "python3" but not "python" (which is the case for modern Ubuntu systems - "python" is python2 and is not installed by default)
   
   **To Reproduce**
   Write a simple function `func1.py`
   
   ```
   from pulsar import Function
   
   class FirstFunction(Function):
       def process(self, item, context):
           log = context.get_logger()
           log.info("Got %r with properties %r" % (item, context.get_message_properties()))
   ```
   
   Deploy it:
   
   ```
   apache-pulsar-2.4.1/bin/pulsar-admin functions create --name womble --inputs my-topic \
     --py /home/ubuntu/func1.py --classname FirstFunction --log-topic pulsar-log
   ```
   
   (side note: have to give absolute path to function, pulsar-admin changes the working directory)
   
   Check its status:
   
   ```
   $ apache-pulsar-2.4.1/bin/pulsar-admin functions status --name womble
   {
     "numInstances" : 1,
     "numRunning" : 0,
     "instances" : [ {
       "instanceId" : 0,
       "status" : {
         "running" : false,
         "error" : "Cannot run program \"python\": error=2, No such file or directory",
         "numRestarts" : 16,
         "numReceived" : 0,
         "numSuccessfullyProcessed" : 0,
         "numUserExceptions" : 0,
         "latestUserExceptions" : [ ],
         "numSystemExceptions" : 0,
         "latestSystemExceptions" : [ ],
         "averageLatency" : 0.0,
         "lastInvocationTime" : 0,
         "workerId" : "c-standalone-fw-ldex-pulsar.int.example.net-8080"
       }
     } ]
   }
   ```
   
   Note error: **Cannot run program "python": error=2, No such file or directory**
   
   **Expected behavior**
   I would expect the function to deploy.
   
   **Screenshots**
   N/A
   
   **Desktop (please complete the following information):**
   N/A
   
   **Additional context**
   This may be the same problem as #5369 - however in that case, it's the shebang line in pulsar's own python code, whereas this one is my own function which is not starting.
   
   I cannot find it stated in Pulsar documentation whether Pulsar supports python2, python3 or both.  However python2 is [very shortly](https://pythonclock.org/) going to be end-of-life.
   
   **Workarounds**
   
   Install python2 and write my code in python2.  I really don't want to do that, with python2 being end-of-life in 2 months' time.
   
   Symlink python to python3.  However in Ubuntu systems, symlinking python to python3 has the potential to cause all sorts of problems. Many applications and scripts depend on `python` meaning `python2` (in which case they will depend on the `python-XXX` packages rather than `python3-XXX`).  More content [here](https://askubuntu.com/questions/320996/how-to-make-python-program-command-execute-python-3#answer-475815).
   
   Ideally pulsar would try python3 and fallback to python.  If pulsar were to be hard-coded to run "python3" instead of "python" that would also be fine IMO, given the impending EOL of python2.

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