File locking in Python--help!

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I wrote my own web-mail client (bluemail) because I wanted a web-mail client that would work along-side PINE and figured I'd just roll my own in Python. I work on it from time to time--mostly when someone requests new features. Right now I'm trying to implement deleting of items in a given folder.

The problem I'm having is a file locking issue. When the user clicks on the INBOX link, it opens up their inbox file (an mbox file in /var/spool/mail/) as the user. That works super. I parse out the mbox and display a bunch of email one-liners. The problem comes in that I can't seem to open the file as r+, lock it, parse it into email, change some of the email (whether by deleting the email from the mbox file or changing the attributes of the email [i.e. marking the email as having been answered]), then write the changed file back to disk without the possibility of having another process (our MTA, for example) slipping in and changing the file beneath me.

Right now, I open the file like this:

   import fcntl

   f = open("/some/file", "r+")
   fcntl.flock(f, fcntl.LOCK_EX)

That should work fine (as far as my research has told me). I did notice some places that say that LOCK_EX only works when you open the file in "a" or "w" mode, but I've got it opened as "r+" which means I'm going to update it later, so I'm puzzled.

Anyhow, using the code above doesn't seem to lock the file. I can run the above code thereby "locking" the file and then in another process open the file, read the file, and write to it. That indicates to me that it's not really locked.

Any ideas how I can resolve this? I need to be able to open a file for reading and updating and lock it so that other processes can't change the file between my read and update cycles. I'm running the code on a Debian GNU/Linux box with Python 2.3.

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