code: def checkPidFile(pidfile):
      
      if os.path.exists(pidfile):
          try:
              with open(pidfile) as f:
                  pid = int(f.read())
          except ValueError:
              raise ValueError('Pidfile {} contains non-numeric value'.format(pidfile))
          try:
              os.kill(pid, 0)
          except OSError as why:
              if why.errno == errno.ESRCH:
                  # The pid doesn't exist.
                  print('Removing stale pidfile {}'.format(pidfile))
                  os.remove(pidfile)
              else:
                  raise OSError("Can't check status of PID {} from pidfile {}: {}".format(
                      pid, pidfile, why))
          else:
              raise BusyError("'{}' exists - is this master still running?".format(pidfile))
docs: mostly comes from _twistd_unix.py which is not twisted public API :-/
  
          except it returns an exception instead of exiting