


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!")
The above is the detailed content of Why Does Python's `subprocess.readlines()` Hang When Streaming Ruby Output, and How Can I Fix It?. For more information, please follow other related articles on the PHP Chinese website!

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