Subprocess Popen confusion

[Peter Otten]

Dick Holmes wrote:

> https://occovid19.ochealthinfo.com/coronavirus-in-oc 

> I'm trying to
> communicate using a continuing dialog between two
> processes on the same system.

I think pexpect

https://pexpect.readthedocs.io/en/stable/index.html

does this naturally, but I don't know if Windows support is sufficient for 
your needs.

> I've looked at various mechanisms and the
> class that seems to fit my needs is Popen in the subprocess module, but
> I can't seem to get more than a single round-trip message through Popen.
> I first call Popen then poll using the identifier returned from the call
> and the poll seems to work. I then call the communicate function passing
> None as the value to send to the companion process stdin. I get the
> expected result, but I also get "Exception condition detected on fd 0
> \\n" and "error detected on stdin\\n". Subsequent attempts to
> read/write/communicate with the subprocess fail because the file (stdxx
> PIPE) is closed.
> 
> I can't tell from the documentation if the communicate function is a
> one-time operation. 

Yes, communicate() is one-off, 


        """Interact with process: Send data to stdin and close it.
        Read data from stdout and stderr, until end-of-file is
        reached.  Wait for process to terminate.
        ...
        """

seems pretty clear. What would you improve?

> I have tried using read but the read call doesn't
> return (I'm using winpdb-reborn to monitor the operations).

Try readline(). Deadlocks may happen ;)
 
> I'm using Python 3.7, Windows 10, winpdb-reborn 2.0.0, rpdb2 1.5.0. If
> it makes any difference, I'm trying to communicate with GDB using the MI
> interpreter.
> 
> Thoughts and advice appreciated!
> 
> Dick