Parse output in real time of any program in Python efficiently

real_time_program_output_parser.py

#!/usr/bin/env python
# To make Python 2.7 print an actual function
from __future__ import print_function
import subprocess
# shlex takes care of splitting a string representing a command in words for subprocess
import shlex
import os


class RealTimeProgramOutputParser(object):
    """
    Given a command and a function to execute, it executes that function for every new line
    from the output of the command.
    If the program ends a RuntimeError exception is raised.

    E.g.: `t = RealTimeProgramOutputParser('dmesg --follow', print)`
    Will print every dmesg line as they come.
    """

    def __init__(self, command, function_for_every_new_line):
        # Run the process
        self._process = subprocess.Popen(shlex.split(command),
                                         shell=False,
                                         # We pipe the output to an internal pipe
                                         stdout=subprocess.PIPE,
                                         # Make sure that if this Python program is killed, this gets killed too
                                         preexec_fn=os.setsid)
        # Poll for output, note: readline() blocks efficiently until there is output with a newline
        while True:
            output = self._process.stdout.readline()
            # Polling returns None when the program is still running, return_code otherwise
            return_code = self._process.poll()
            if return_code is not None:
                # Program ended, get exit/return code
                raise RuntimeError("Command '{}' finished with exit code {}".format(command, return_code))

            # If the output is not empty, feed it to the function, strip the newline first
            if output:
                function_for_every_new_line(output.strip())


if __name__ == '__main__':
    rtpop = RealTimeProgramOutputParser('dmesg --follow', print)

Got to ask. Why is this (if it is) any better or faster than using fileinput.input() then piping command output to python script?

import fileinput

with fileinput.input() as f:
    for i in f:
        print(i)
        # or do something else instead print

Just trying to see pros and cons for using something like this to parse log file live and act upon when some conditions are met.

I've never used fileinput, so I don't know!