Using Python to run executable and fill in user input

Question

I'm trying to use Python to automate a process that involves calling a Fortran executable and submitting some user inputs. I've spent a few hours reading through similar questions and trying different things, but haven't had any luck. Here is a minimal example to show what I tried last

#!/usr/bin/python

import subprocess

# Calling executable 
ps = subprocess.Popen('fortranExecutable',shell=True,stdin=subprocess.PIPE)
ps.communicate('argument 1')
ps.communicate('argument 2')

However, when I try to run this, I get the following error:

  File "gridGen.py", line 216, in <module>
    ps.communicate(outputName)
  File "/opt/apps/python/epd/7.2.2/lib/python2.7/subprocess.py", line 737, in communicate
    self.stdin.write(input)
ValueError: I/O operation on closed file

Any suggestions or pointers are greatly appreciated.

EDIT:

When I call the Fortran executable, it asks for user input as follows:

fortranExecutable
Enter name of input file: 'this is where I want to put argument 1'
Enter name of output file: 'this is where I want to put argument 2'

Somehow, I need to run the executable, wait until it asks for user input and then supply that input.

Solution 1

If the input doesn't depend on the previous answers then you could pass them all at once using .communicate():

import os
from subprocess import Popen, PIPE

p = Popen('fortranExecutable', stdin=PIPE) #NOTE: no shell=True here
p.communicate(os.linesep.join(["input 1", "input 2"]))

.communicate() waits for process to terminate therefore you may call it at most once.

Solution 2

As the spec says communicate() awaits for the subprocess to terminate, so the second call will be addressed to the finished process.

If you want to interact with the process, use p.stdin&Co instead (mind the deadlock warning).