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

Stopping Process Output Reading in Python Without Hangs

Background

When using Python's os.popen() function with tools that produce continuous output, the program often hangs when trying to read the output.

os.popen() Issue

The problematic line process = os.popen("top").readlines() halts the program due to readlines(), which attempts to read the entire process output at once.

Solution with subprocess.Popen()

To resolve this issue, use subprocess.Popen() instead of os.popen(). Here's a corrected example:

<code class="python">import subprocess
import time
import os

# Start "top" process with stdout redirection
process = subprocess.Popen(["top"], stdout=subprocess.PIPE)

# Wait for 2 seconds
time.sleep(2)

# Send kill signal to "top" process
os.popen("killall top")

# Read process output
output, _ = process.communicate()
print(output.decode())</code>

This modified code:

• Captures the process output in a variable using communicate() instead of readlines().
• Sends the kill signal to the "top" process.
• Declares end-of-file for the process's I/O stream and exits the program.

Tail-like Approach

If you only need a portion of the process output, you can use a tail-like solution to capture a specific number of lines.

Thread-based Approach

To capture process output in a separate thread, try the following:

<code class="python">import collections
import subprocess
import threading

# Start process with stdout redirection
process = subprocess.Popen(["top"], stdout=subprocess.PIPE)

# Define function to read process output in a thread
def read_output(process):
    for line in iter(process.stdout.readline, ""):
        ...  # Implement your logic here to process each line

# Create and start a thread for reading and processing output
reading_thread = threading.Thread(target=read_output, args=(process,))
reading_thread.start()

# Wait for 2 seconds, then terminate the process
time.sleep(2)
process.terminate()

# Wait for the reading thread to complete
reading_thread.join()</code>

signal.alarm() Approach

You can also use signal.alarm() to terminate the process after a specified timeout:

<code class="python">import collections
import signal
import subprocess

# Define signal handler
def alarm_handler(signum, frame):
    # Raise an exception to terminate the process reading
    raise Exception

# Set signal handler and alarm for 2 seconds
signal.signal(signal.SIGALRM, alarm_handler)
signal.alarm(2)

# Start process with stdout redirection
process = subprocess.Popen(["top"], stdout=subprocess.PIPE)

# Capture process output
number_of_lines = 200
q = collections.deque(maxlen=number_of_lines)
for line in iter(process.stdout.readline, ""):
    q.append(line)

# Cancel alarm
signal.alarm(0)

# Print captured output
print(''.join(q))</code>

threading.Timer Approach

Alternatively, you can use threading.Timer to schedule the process termination:

<code class="python">import collections
import subprocess
import threading

# Define function to terminate the process
def terminate_process(process):
    process.terminate()

# Start process with stdout redirection
process = subprocess.Popen(["top"], stdout=subprocess.PIPE)

# Create and start a timer to terminate process in 2 seconds
timer = threading.Timer(2, terminate_process, [process])
timer.start()

# Capture process output
number_of_lines = 200
q = collections.deque(process.stdout, maxlen=number_of_lines)

# Cancel timer
timer.cancel()

# Print captured output
print(''.join(q))</code>

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