Making a python script only be able to run once at a time [duplicate]

This question already has answers here: 开发者_JAVA技巧 Closed 10 years ago.

Possible Duplicate:

Python: single instance of program

I am looking to make a python script be unique in the sense that it can only run once at a time. For example if I run the script and open another session of the same script a second time and the first session is still running, then the second session will just exit and do nothing. Anyone knows how I could implement this?

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Never written python before, but this is what I've just implemented in mycheckpoint, to prevent it being started twice or more by crond:

import os
import sys
import fcntl
fh=0
def run_once():
    global fh
    fh=open(os.path.realpath(__file__),'r')
    try:
        fcntl.flock(fh,fcntl.LOCK_EX|fcntl.LOCK_NB)
    except:
        os._exit(0)

run_once()

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One poor man's solution is to use a file based lock. If you open a file using os.open(), there is a flag that allows an exclusive lock on the file. See this for reference.