passthru.py

import errno
import os
import subprocess
import sys

# Run command and immediately print output as it is generated
def passthru(cmd):
    # Create stdout and stderr pseudoterminal pairs
    stdout_master, stdout_slave = os.openpty()
    stderr_master, stderr_slave = os.openpty()
    # Run the command redirecting the output to slave ends of ptys
    p = subprocess.Popen(cmd, stdout=stdout_slave, stderr=stderr_slave)
    # Close the slave ends on python's end as they have been passed to the process
    os.close(stdout_slave)
    os.close(stderr_slave)
    # Redirect master ends to python's stdout / stderr
    readable = {stdout_master: sys.stdout.buffer, stderr_master: sys.stderr.buffer}
    while readable:
        # Wait for descriptors to be ready
        for fd in select.select(readable, [], [])[0]:
            try:
                # Read the data from the process and write them to python's stdout / stderr
                data = os.read(fd, 1024)
                readable[fd].write(data)
                readable[fd].flush()
            except OSError as e:
                # Remove the file descriptor on EOF
                if e.errno != errno.EIO:
                    raise
                del readable[fd]
    # Close the master ends and return the return code of the finished process
    os.close(stdout_master)
    os.close(stderr_master)
    return p.poll()