control stdin and stdout of a ruby program in python

Question

First I should notice: I'm a python programmer with no knowledge about ruby!

Now, I need to feed stdin of a ruby program and capture stdout of the script with a python program.
I tried this (forth solution) and the code works in python2.7 but not in python3; The python3 code reads input with no output.

Now, I need a way to tie the ruby program to either python 2 or 3.

My try:

This code written with six module to have cross version compatibility.

python code:

from subprocess import Popen, PIPE as pipe, STDOUT as out

import six

print('launching slave')
slave = Popen(['ruby', 'slave.rb'], stdin=pipe, stdout=pipe, stderr=out)

while True:
    if six.PY3:
        from sys import stderr
        line = input('enter command: ') + '\n'
        line = line.encode('ascii')
    else:
        line = raw_input('entercommand: ') + '\n'
    slave.stdin.write(line)
    res = []
    while True:
        if slave.poll() is not None:
            print('slave rerminated')
            exit()
        line = slave.stdout.readline().decode().rstrip()
        print('line:', line)
        if line == '[exit]': break
        res.append(line)
    print('results:')
    print('\n'.join(res))

ruby code:

while cmd = STDIN.gets
    cmd.chop!
    if cmd == "exit"
        break
    else
        print eval(cmd), "\n"
        print "[exit]\n"
        STDOUT.flush
    end
end

NOTE:

Either another way to do this stuff is welcomed! (like socket programming, etc.)
Also I think it's a better idea to not using pipe as stdout and use a file-like object. (like tempfile or StringIO or etc.)

score 1 · Accepted Answer · answered Sep 27 '14 at 06:38

It's because of bufsize. In Python 2.x, default value was 0 (unbufffered). And in Python 3.x it changed to -1 (using default buffer size of system).

Specifying it explicitly will solve your problem.

slave = Popen(['ruby', 'slave.rb'], stdin=pipe, stdout=pipe, stderr=out, bufsize=0)

DEMO

score 0 · Answer 2 · answered Jul 09 '21 at 11:04

Below is the code on how I got it working with Ruby & Python3.

Ruby Slave

# read command from standard input:
while cmd = STDIN.gets
  # remove whitespaces:
  cmd.chop!
  # if command is "exit", terminate:
  if cmd == "exit"
    break
  else
    # else evaluate command, send result to standard output:
    print eval(cmd), "\n"
    print "[exit]\n"
    # flush stdout to avoid buffering issues:
    STDOUT.flush
  end
end

Python master

from subprocess import Popen, PIPE as pipe, STDOUT as out

print('Launching slave')
slave = Popen(['ruby', 'slave.rb'], stdin=pipe, stdout=pipe, stderr=out, bufsize=0)

while True:
    from sys import stderr
    line = input('Enter command: ') + '\n'
    line = line.encode('ascii')
    slave.stdin.write(line)
    res = []
    while True:
        if slave.poll() is not None:
            print('Slave terminated')
            exit()
        line = slave.stdout.readline().decode().rstrip()
        if line == '[exit]': break
        res.append(line)
    print('results:')
    print('\n'.join(res))

control stdin and stdout of a ruby program in python

My try:

NOTE:

2 Answers2