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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: Monitoring updating directory for image for GUI
Date: Fri, 08 Feb 2013 20:32:29 -0500
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On Fri, 8 Feb 2013 09:09:31 -0800 (PST), ciscorucinski@gmail.com
declaimed the following in gmane.comp.python.general:

> 
> That is what I am trying to figure out how to do. I have a counter that updates with every note that is grabbed from the stream. That counter is passed into a threaded class that might or might not create the image right away...but seems to create them sequentially from what I have seen so far (I don't know if that is absolute thought). 
> 
> Please let me know you thought on the above part! Each new thread should only take only a very small amount longer to do than the previous thread that was started...because it has one more note to deal with this time.
> 
> Anyways...I do have the count of the latest image that WILL BE created...but by the time that one is created in the directory, a new note is most likely streamed in and replaces that count. Therefore, I cannot really do that...I think.
> 
> hmmmm, I wonder. I create the images via OS command that will .wait() on a subprocess.Popen(). I could increment the counter there? Or do something there that will let me know of the most recent image file via a counter or something? But it needs to be thread-safe
>

	One problem I see is a race condition. Unless you have some sort of
completion flag from the program generating the images you can't rely on
the "newest" file in the directory being a complete image.

	So... You either run one image behind if just looking at directory
contents (on the basis the newest image file is still being written); or
you have to run lock-step with the image generator, where it signals
when <file-n> is available.

	If monitoring a directory for changes, and you are on Windows, I
believe there is a native Win32 API to catch changes to directories.
{I'm still waiting for Windows Internals Part 2 to arrive -- though this
series is closer to the old VMS "black book" than to a reference manual
for Windows system calls}

http://msdn.microsoft.com/en-us/library/windows/desktop/aa365261%28v=vs.85%29.aspx

	You still have the chase condition unless you code a two level
change... 

	displayFile = None
	inWorkFile = None
	while True:
		#wait for change notification of new file (ignore times, etc.)
		displayFile = inWorkFile
		inWorkFile = #whatever the change notification reported
		if displayFile:
			#do something with completed file

... and that leaves the matter of handling the last file when you
conclude there will be no more updates.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/