singular.py

import os
import sys

def taskexists(pid):
    if sys.platform == "win32":
        import ctypes
        import win32con
        
        h = ctypes.windll.kernel32.OpenProcess(win32con.PROCESS_ALL_ACCESS, False, pid)
        
        if h:
            return True
        
        return False
    else:
        return os.kill(pid, 0)

def checkInstance(pidfile):
    if os.path.isfile(pidfile) and os.access(pidfile, os.F_OK):
        with open(pidfile, "r") as f:
            pid = f.read()
            try:
                pid = int(pid)
            except:
                print "Bad pid: %s" % pid
                return False
                
            if taskexists(pid):
                print "Already exists!"
                return True

        os.unlink(pidfile)
        return False
                
if __name__ == "__main__":
    pidfile = "socbot.pid"

    if not checkInstance(pidfile):
        print "FIRST"
        pid = str(os.getpid())
        
        with open(pidfile, 'w') as f:
            f.write(pid)
        
        # Do some actual work here
    
        #os.unlink(pidfile)