How to Tail Log Files in Python: Blocking vs. Non-Blocking?

Tailing Log Files with Python

One often wants to tail a log file or otherwise passively watch it to view the new content added to it. This can be achieved in Python in a few ways.

Non-Blocking

For this approach, one can use the subprocess and select modules.

import time
import subprocess
import select

f = subprocess.Popen(['tail', '-F', filename],
                      stdout=subprocess.PIPE, stderr=subprocess.PIPE)
p = select.poll()
p.register(f.stdout)

while True:
    if p.poll(1):
        print(f.stdout.readline())
    time.sleep(1)

This code opens a subprocess running tail -F on the specified file and polls its output for new data, printing it when available. This approach doesn't block the main program.

Blocking

A simpler blocking approach is available using only the subprocess module.

import subprocess

f = subprocess.Popen(['tail', '-F', filename],
                      stdout=subprocess.PIPE, stderr=subprocess.PIPE)
while True:
    line = f.stdout.readline()
    print(line)

This code also prints new lines as they are added, but it will block until the tail program is closed.

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