usage: with singleprocess('/tmp/test.pid'): pass

singleprocess.py

import time
class singleprocess:
    def __init__(self, pidfile):
        self.pidfile = pidfile
    def __enter__(self):
        pid = str(os.getpid())
        if os.path.isfile(self.pidfile+'.0'):
            self.realpidfile = self.pidfile+'.1'
            if os.path.isfile(self.pidfile+'.1'):
                raise Exception("%s already exists, exiting" % self.realpidfile)
            else:
                with open(self.realpidfile, 'w') as file: file.write(pid)
                while os.path.isfile(self.pidfile+'.0'):
                    time.sleep(1)
                    print 'wait'
        else:
            self.realpidfile = self.pidfile+'.0'
            with open(self.realpidfile, 'w') as file: file.write(pid)
            while os.path.isfile(self.pidfile+'.1'):
                time.sleep(1)
                print 'wait'
    def __exit__(self, type, value, traceback):
        os.unlink(self.realpidfile)