Running an external executable in Python interactively

Question

A simple C program is compiled using gcc test.c -o test.exe :

        /* TEST.C */
#include <stdio.h>
#include <string.h>
#define TRUE 1

int main (void)
{
  char p[256];
  strcpy(p, "start\0");
  printf("Welcome!\n");
  while (TRUE)
  {
    printf("Input: ");
    if (fgets(p, 255, stdin) == NULL)  break;
    if (p[0] == 'q')                   break;
    printf("You Entered: %s\n", p);
  }
  return 0;
}

The subprocess module in Python portends to start a program and grab its stdin and stdout . Simple test using the above program:

import subprocess
from subprocess import PIPE, STDOUT

output = open("test.out","w")
ris = subprocess.Popen(executable="test.exe", args="", stdin=PIPE,
                       stdout=output, stderr=STDOUT,
                       universal_newlines=True, shell=True)  
com = ris.communicate(input='bye\n')
com = ris.communicate(input='q\n')
output.close()

appears to loop infinitely on the first communicate command, filling test.out with:

Welcome!
You Entered: bye
You Entered: bye
You Entered: bye
You Entered: bye
.
.
.

until the user interrupts the python process. Using ris.stdin.write('bye\\n') instead of communicate does not seem to send input at all even when followed by ris.stdin.flush() .

What is the proper way to interactively run an executable with subprocess ? The intention is to leave a single instance of the program running in the background while multiple inputs are passed in through stdin and the corresponding outputs are read from stdout or stderr

Answer 1

In addition to the errors in your C program that have been mentioned, you are calling .communicate() more than once. .communicate() is single-use function that sends all of its data to the subprocess and waits for the subprocess to exit.

If you must use .communicate() , use it this way:

com = ris.communicate(input='bye\nq\n')

Alternatively, you can use ris.stdin.write() , like so:

import subprocess
from subprocess import PIPE, STDOUT

output = open("test.out","w")
ris = subprocess.Popen(executable="/tmp/test.exe", args="", stdin=PIPE,
                       stdout=output, stderr=STDOUT,
                       universal_newlines=True, shell=True)
com = ris.stdin.write('bye\n')
com = ris.stdin.write('q\n')
ris.stdin.flush()
output.close()
ris.wait()