How to Avoid Python Programs from Hanging When Reading Process Output?-Python Tutorial-php.cn

How to Avoid Python Programs from Hanging When Reading Process Output?

Susan Sarandon

Release： 2024-11-02 13:54:29

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How to Avoid Python Programs from Hanging When Reading Process Output?

Stop reading process output in Python without hang?

Problem:

A Python program needs to interact with an external process (e.g., "top") that continuously produces output. However, simply reading the output directly can cause the program to hang indefinitely.

Solution:

To prevent hanging, it's essential to employ non-blocking or asynchronous mechanisms when reading process output. Here are a few possible approaches:

Spooled Temporary File (Recommended)

This method utilizes a dedicated file object to store the process output.

#!/usr/bin/env python<br>import subprocess<br>import tempfile<br>import time</p>
<p>def main():</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"># Open a temporary file (automatically deleted on closure)
f = tempfile.TemporaryFile()

# Start the process and redirect stdout to the file
p = subprocess.Popen(["top"], stdout=f)

# Wait for a specified duration
time.sleep(2)

# Kill the process
p.terminate()
p.wait()

# Rewind and read the captured output from the file
f.seek(0)
output = f.read()

# Print the output
print(output)
f.close()

Copy after login

This approach employs a separate thread to continuously read the process output while the main thread proceeds with other tasks.

import collections import subprocess import threading import time def read_output(process, append): <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">for line in iter(process.stdout.readline, ""): append(line)

# Start the process and redirect stdout process = subprocess.Popen(["top"], stdout=subprocess.PIPE, close_fds=True) # Create a thread for output reading q = collections.deque(maxlen=200) t = threading.Thread(target=read_output, args=(process, q.append)) t.daemon = True t.start() # Wait for the specified duration time.sleep(2) # Print the saved output print(''.join(q))

import collections import signal import subprocess class Alarm(Exception): <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">pass

# Start the process and redirect stdout process = subprocess.Popen(["top"], stdout=subprocess.PIPE, close_fds=True) # Set signal handler signal.signal(signal.SIGALRM, alarm_handler) signal.alarm(2) try: # Read and save a specified number of lines q = collections.deque(maxlen=200) for line in iter(process.stdout.readline, ""): q.append(line) signal.alarm(0) # Cancel alarm except Alarm: process.terminate() finally: # Print the saved output print(''.join(q))

import collections import subprocess import threading def main(): <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"># Start the process and redirect stdout process = subprocess.Popen(["top"], stdout=subprocess.PIPE, close_fds=True) # Create a timer for process termination timer = threading.Timer(2, process.terminate) timer.start() # Read and save a specified number of lines q = collections.deque(maxlen=200) for line in iter(process.stdout.readline, ""): q.append(line) timer.cancel() # Print the saved output print(''.join(q))

import collections import subprocess import sys import time def main(): <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">args = sys.argv[1:] if not args: args = ['top'] # Start the process and redirect stdout process = subprocess.Popen(args, stdout=subprocess.PIPE, close_fds=True) # Save a specified number of lines q = collections.deque(maxlen=200) # Set a timeout duration timeout = 2 now = start = time.time() while (now - start) < timeout: line = process.stdout.readline() if not line: break q.append(line) now = time.time() else: # On timeout process.terminate() # Print the saved output print(''.join(q))