Issues with threading and subprocess in Python 2.7 causing unexpected behavior
I'm migrating some code and I'm a bit lost with I’m encountering unexpected behavior when using the `subprocess` module in combination with Python's `threading` library in Python 2.7. Specifically, I’m trying to run a long-running shell command in a separate thread and capture its output, but it appears that the output is getting mixed up or truncated intermittently. Here’s a simplified version of what I’ve tried: ```python import threading import subprocess class CommandThread(threading.Thread): def __init__(self, command): threading.Thread.__init__(self) self.command = command self.output = None def run(self): process = subprocess.Popen(self.command, stdout=subprocess.PIPE, stderr=subprocess.PIPE) self.output, _ = process.communicate() cmd_thread = CommandThread(['ping', '-c', '5', 'google.com']) cmd_thread.start() cmd_thread.join() print(cmd_thread.output) ``` The command I’m running is a simple ping command, but in my actual application, it’s a more complex script that might output information at irregular intervals. The issue arises when I try to execute the command multiple times in quick succession; sometimes the output is incomplete or mixed. I suspect the global interpreter lock (GIL) or the way the subprocess handles output in a separate thread may be the culprit. I’ve tried using `threading.Lock()` to synchronize access to the output variable, but it hasn’t resolved the issue. Additionally, I looked into using `subprocess.PIPE` for stderr to catch any errors, but they seem to get lost too. Here’s the relevant part of my code that uses the lock: ```python class CommandThread(threading.Thread): def __init__(self, command, lock): threading.Thread.__init__(self) self.command = command self.output = None self.lock = lock def run(self): process = subprocess.Popen(self.command, stdout=subprocess.PIPE, stderr=subprocess.PIPE) out, err = process.communicate() with self.lock: self.output = out ``` Using the lock seems to help a bit, but I still notice that the output is sometimes cut off or appears in a jumbled manner. Has anyone faced a similar issue with threading and subprocess in Python 2.7? Any suggestions on how to properly capture the output without losing data? This is part of a larger web app I'm building. I'm using Python stable in this project.