from subprocess import Popen, PIPE
from signal import alarm, signal, SIGALRM

class Alarm(Exception):
    pass

def handler(signum, frame):
    # map the signal to an exception
    raise Alarm

def dnssd(params, pattern='local.', timeout=3):
    h = Popen('dns-sd %s' % params, shell=True, stdout=PIPE)

    # set up a timeout
    if timeout:
        signal(SIGALRM, handler)
        alarm(timeout)

    result = []

    try:
        # readline will block when dns-sd enters its loop
        while True:
            line = h.stdout.readline()
            if pattern in line:
                result.append(line)
    except Alarm:
        h.kill()
    return result

# grab all instances of _ipp._tcp services
printers = map( lambda entry: entry.split('_ipp._tcp.')[1].strip(),
                dnssd('-B _ipp._tcp.', 'local.', 3) )

# tack on 'URF=none' to each service info
entries = []
for p in printers:
    entries.extend(map( lambda record: (p.split("@")[0].split(' ')[0],
                        record.strip() + ' URF=none'),
                        dnssd('-L "%s" _ipp' % p, 'txtvers', 1) ))

# now advertise the first entry (advertising more than one would require
# something like multiprocessing.Pool() or just plain exec(), and I have
# just the one printer...)
dnssd('-R "%s AirPrint" _ipp._tcp,_universal local. 631 %s' % entries[0],
      timeout=0)