How can I avoid running a python script multiple times?

[Cameron Laird]

In article <3E3677E5.5070901 at Linux.ie>,  <Padraig at Linux.ie> wrote:
>Grzegorz Adam Hankiewicz wrote:
>> Hi.
>> 
>> Using the concurrency jargon, I'm trying to mark python scripts as
>> 'critical processes' which are not allowed to be run more than one
>> at the same time (I'm constrained to unix environments, BTW). To
>> do so, and following the logic of a bash script I had, I've created
>> a function which is passed the real entry point of the script.
>
>[snip]
>
>> def run_if_possible(entry_point):
>>    """Given a function, it is called if there's no previous process
>running"""
>>    permission = 0
>>    lock_path = "%s.lock" % os.path.realpath(sys.argv[0])
>>    if os.path.isfile(lock_path):
>>       file = open(lock_path, "rt")
>
>what does t do in "rt" ?
>Hmm this didn't give an error for me: file = open(lock_path, "wqst")
>So unrecognised options must be ignored. I tried x also, but this
>seemed to set the exclusive bit? Where are these documented?
>Ah, OK on Linux (glibc):
>     info libc "Opening Streams"
>This shows that the x is glibc specific and sets O_EXCL
>on the subsequent open() call (which is what you want).
>
>You could just do os.system("sematree ...") as I've worked
>this out already: http://www.pixelbeat.org/sematree/
>
>Note a simple atomic locking mechanism could be to
>try and create a directory, which should work over
>NFS etc. Might be simpler that messing with exclusive
>bits on files?
>
>Note also sockets would be troublesome as you'll
>have issues with SO_REUSEADDR for one thing.
>
>Pádraig.
>

For Unix, or multiple platforms centered on Unix, my favorite
singleton resource manager pertinent to this discussion is the
socket service.  <URL: http:/wiki.tcl.tk/singleton > has a 
discussion that's easily adapted to Python.
-- 

Cameron Laird <Cameron at Lairds.com>
Business:  http://www.Phaseit.net
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