Pass integer input argument for Popen.communicate()

Question

I am pretty new to python and I am stuck with something now. I am using python 2.7. I am trying to automate a command in shell for which I wrote a python script. I have to input

Answer 1

Pipes are byte streams in Unix. There is no numeric input -- there is only "string" input (there are no numbers or anything else -- there are only bytes).

If your child java process expects a number as its ascii representation then .communicate(input='2') is correct already if the subprocess reads from stdin (System.in). If the java process reads from the console directly (System.console().readLine()) then stdin=PIPE won't work (you might need to provide a pty in this case e.g., using pexpect).

For example, to read an integer from stdin and print it back in Java:

import java.util.Scanner;

public class ReadInteger
{
  public static void main(String args[])
  {
    Scanner in = new Scanner(System.in);
    int i = in.nextInt();
    System.out.println("got " + i);
  }
}

To pass an integer from Python process to the Java process via a pipe:

#!/usr/bin/env python2
from subprocess import Popen, PIPE

p = Popen(['java', '-ea', 'ReadInteger'], stdin=PIPE, stdout=PIPE)
output = p.communicate(input='2')[0]
print 'from java {' + output.strip() + '}'

It works (the output is from java {got 2}).

It stops working if the child process reads from the console instead:

public class ReadIntegerConsole
{
  public static void main(String args[])
  {
    int i = Integer.parseInt(System.console().readLine());
    System.out.println("got " + i);
  }
}

The same python script leads to NullPointerException (and the output is from java {}) because System.console() is null here (stdin is redirected). It works if we provide a pseudo-tty:

#!/usr/bin/env python2
import pexpect # $ pip install pexpect

child = pexpect.spawn('java -ea ReadIntegerConsole')
child.setecho(False)
child.sendline('2')
child.expect(pexpect.EOF)
child.close()
print "from java {" + child.before.strip() + "}"

It works (the output is from java {got 2}).

Answer 2

Finally I found the solution for this. Well the solution to this was pretty simple

---

when I changed

string = proc.communicate(input='2')

to

string = proc.communicate(input='2\n')

it worked fine.

Answer 3

The ./sbt process you are calling needs to be able to read a string and convert it into a integer. It is not possible to send native Python types to a child process with only Popen.

Another potential solution would be to serialize the data into JSON, YAML, XML, python pickle format, etc - choose your standard. However as you are passing a single integer this would seem very much overkill.