locking files

[Joseph Holland King]

is there anyway to lock a file so that two different programs cannot access
the file at the same time. ie, my python module is editting file foo, and
at the same time user x (or program x) tries to read from it. is there 
anyway that the python module can prevent x from accessing the file for
a given amount of time?

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Joseph Holland King  | "God whispers to us in our pleasures, speaks in our
                     |  conscience, but shouts in our pains: it is His
                     |  megaphone to rouse a deaf world." C. S. Lewis