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)

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