tee daemon won't exit

a.sh

#!/bin/bash

main() {
    echo 1
    python sig.py &
    echo 2
}

main 2>&1 | tee -a a.log

sig.py

import os
import sys
import signal

def handle(signum, frame):
    if signum in [1, 2, 3, 15]:
        print('exiting with sig: %s' % signum)
        sys.exit()
    else:
        print('sig %s, ignore' % signum)


def main():
    bypass = ['SIG_DFL', 'SIGKILL', 'SIGSTOP']

    for i in [x for x in dir(signal) if x.startswith('SIG')]:
        if not i in bypass:
            signum = getattr(signal, i)
            signal.signal(signum, handle)
            print(signum, i)

    print('pid: %s, ppid: %s' % (os.getpid(), os.getppid()))
    while 1: pass


if __name__ == '__main__':
    try:
        pid = os.fork()
        if pid > 0:
            sys.exit(0)
    except OSError, e:
        print >> sys.stderr, 'fork #1 failed: %d (%s)' % (e.errno, e.stderror)

    os.chdir('/')
    os.setsid()
    os.umask(0)

    try:
        pid = os.fork()
        if pid > 0:
            print 'daemon PID %d' % pid
            sys.exit(0)

    except OSError, e:
        print >>sys.stderr, 'fork #2 failed: %d (%s)' % (e.errno, e.stderror)

    main()