How to Prevent Python from Freezing When Reading Output from a Constantly Updating Process?

Stopping Reading Process Output in Python Without Hanging

When attempting to read the output of a constantly updating process in Python, you may encounter freezing issues. This problem arises specifically when using the readlines() method.

Problem: Freezing When Using readlines()

In the provided code snippet, the process = os.popen("top").readlines() line causes the program to hang. This is because readlines() reads the entire output of the subprocess at once, which can be substantial and result in a long blocking operation.

Using subprocess.Popen()

A better approach is to use the subprocess.Popen() function to create a subprocess and manage its input and output. Here's how you can do it:

<code class="python">import subprocess

process = subprocess.Popen('top')
time.sleep(2)
os.popen("killall top")
print process</code>

This code uses Popen() to create a subprocess running top. After waiting for 2 seconds, it terminates the top process, and finally prints the subprocess object. However, this approach still returns an unformatted object.

Solution Using a Temporary File

To read the output of the subprocess without blocking the program, you can use a temporary file to store the output. Here's an improved version of the code:

<code class="python">#!/usr/bin/env python
import subprocess
import tempfile
import time

def main():
    # Create a temporary file to store the subprocess output
    f = tempfile.TemporaryFile()

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

    # Wait for a few seconds
    time.sleep(2)

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

    # Seek to the start of the temporary file and read the output
    f.seek(0)
    output = f.read()

    # Close the temporary file
    f.close()

    # Print the output of the subprocess
    print(output)

if __name__=="__main__":
    main()</code>

This solution ensures that the program does not hang while reading the output of the subprocess. The subprocess.wait() method waits for the subprocess to terminate, ensuring that all the output has been written to the file before accessing it.

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