Python subprocess readlines() hangs

Question

The task I try to accomplish is to stream a ruby file and print out the output. (NOTE: I don't want to print out everything at once)

main.py

from subprocess import Popen, PIPE, STDOUT

import pty
import os

file_path = '/Users/luciano/Desktop/ruby_sleep.rb'

command = ' '.join(["ruby", file_path])

master, slave = pty.openpty()
proc = Popen(command, bufsize=0, shell=True, stdout=slave, stderr=slave, close_fds=True)     
stdout = os.fdopen(master, 'r', 0)

while proc.poll() is None:
    data = stdout.readline()
    if data != "":
        print(data)
    else:
        break

print("This is never reached!")

ruby_sleep.rb

puts "hello"

sleep 2

puts "goodbye!"

Problem

Streaming the file works fine. The hello/goodbye output is printed with the 2 seconds delay. Exactly as the script should work. The problem is that readline() hangs in the end and never quits. I never reach the last print.

I know there is a lot of questions like this here a stackoverflow but non of them made me solve the problem. I'm not that into the whole subprocess thing so please give me a more hands-on/concrete answer.

Regards

edit

Fix unintended code. (nothing to do with the actual error)

Answer 1

I assume you use pty due to reasons outlined in Q: Why not just use a pipe (popen())? (all other answers so far ignore your "NOTE: I don't want to print out everything at once").

pty is Linux only as said in the docs:

Because pseudo-terminal handling is highly platform dependent, there is code to do it only for Linux. (The Linux code is supposed to work on other platforms, but hasn’t been tested yet.)

It is unclear how well it works on other OSes.

You could try pexpect:

import sys
import pexpect

pexpect.run("ruby ruby_sleep.rb", logfile=sys.stdout)

Or stdbuf to enable line-buffering in non-interactive mode:

from subprocess import Popen, PIPE, STDOUT

proc = Popen(['stdbuf', '-oL', 'ruby', 'ruby_sleep.rb'],
             bufsize=1, stdout=PIPE, stderr=STDOUT, close_fds=True)
for line in iter(proc.stdout.readline, b''):
    print line,
proc.stdout.close()
proc.wait()

Or using pty from stdlib based on @Antti Haapala's answer:

#!/usr/bin/env python
import errno
import os
import pty
from subprocess import Popen, STDOUT

master_fd, slave_fd = pty.openpty()  # provide tty to enable
                                     # line-buffering on ruby's side
proc = Popen(['ruby', 'ruby_sleep.rb'],
             stdin=slave_fd, stdout=slave_fd, stderr=STDOUT, close_fds=True)
os.close(slave_fd)
try:
    while 1:
        try:
            data = os.read(master_fd, 512)
        except OSError as e:
            if e.errno != errno.EIO:
                raise
            break # EIO means EOF on some systems
        else:
            if not data: # EOF
                break
            print('got ' + repr(data))
finally:
    os.close(master_fd)
    if proc.poll() is None:
        proc.kill()
    proc.wait()
print("This is reached!")

All three code examples print 'hello' immediately (as soon as the first EOL is seen).

---

^{leave the old more complicated code example here because it may be referenced and discussed in other posts on SO}

Or using pty based on @Antti Haapala's answer:

import os
import pty
import select
from subprocess import Popen, STDOUT

master_fd, slave_fd = pty.openpty()  # provide tty to enable
                                     # line-buffering on ruby's side
proc = Popen(['ruby', 'ruby_sleep.rb'],
             stdout=slave_fd, stderr=STDOUT, close_fds=True)
timeout = .04 # seconds
while 1:
    ready, _, _ = select.select([master_fd], [], [], timeout)
    if ready:
        data = os.read(master_fd, 512)
        if not data:
            break
        print("got " + repr(data))
    elif proc.poll() is not None: # select timeout
        assert not select.select([master_fd], [], [], 0)[0] # detect race condition
        break # proc exited
os.close(slave_fd) # can't do it sooner: it leads to errno.EIO error
os.close(master_fd)
proc.wait()

print("This is reached!")