How Can I Get Live Output from a Subprocess Command in Python?

Live Output from Subprocess Command

In Python, you can capture the output from a subprocess command and simultaneously produce a live-streaming output using the following approaches:

Using an Iterator

import subprocess
import sys

p = subprocess.Popen(['command'], stdout=subprocess.PIPE)
for line in iter(lambda: p.stdout.readline(1), ''):
    sys.stdout.buffer.write(line)

Using a FileWriter and FileReader

import io
import time
import subprocess
import sys

log = 'test.log'
with io.open(log, 'wb') as writer, io.open(log, 'rb', 1) as reader:
    p = subprocess.Popen(['command'], stdout=writer)
    while p.poll() is None:
        sys.stdout.write(reader.read())
        time.sleep(0.5)
    # Read the remaining
    sys.stdout.write(reader.read())

Original Code Refactoring

In your original code, you can capture the live output after creating a subprocess as follows:

ret_val = subprocess.Popen(run_command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True )
while not ret_val.poll():
    log_file.flush()
    for line in iter(lambda: ret_val.stdout.readline(1), ''):
        if line:
            print(line)
            log_file.write(line.decode())

This method allows you to capture both the stdout and stderr streams and print the live output while simultaneously writing it to a log file.

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