Why Does Python's `subprocess.readlines()` Hang When Streaming Ruby Output, and How Can I Fix It?

Python Subprocess Readlines() Hangs

Problem Statement:

When attempting to stream a Ruby file line by line in Python using the subprocess module, the readlines() call blocks indefinitely, preventing further execution.

Cause:

This issue can arise when using the pty module on a non-Linux operating system to simulate a pseudo terminal. pty is a Linux-specific library, and its behavior on other systems is not guaranteed.

Solutions:

1. Use Pexpect:

Pexpect is a cross-platform library designed for automating interactive applications. It provides a high-level interface for sending and receiving data over a pseudo terminal.

import pexpect

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

2. Use Stdbuf:

Stdbuf can be used to enable line buffering in non-interactive mode, allowing output to be flushed on every line.

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()

3. Use Pty from Standard Library (for Linux):

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!")

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