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()

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if name == "__main__":

main()

Thread-based Output Reading

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

import collections<br>import subprocess<br>import threading<br>import time</p>
<p>def read_output(process, append):</p>
<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)

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