win32 How to make sure a file is completely written?

[justind]

Hello,

I'm using http://code.activestate.com/recipes/156178/ to watch a
folder in windows. It's working perfectly, but sometimes when I try to
open the file immediately after receiving the event, it's not ready to
be opened--if I try to open it with PIL I get IOError: cannot
identify image file and if I try it with a text file, it's empty.
This doesn't happen all the time, just occasionally. I think the
problem is that the file isn't completely written because if I make
the script sleep for a second, it works every time. But that doesn't
seem very elegant or robust.

What's the proper way to make sure the file is ready to be read?

I'm just passing the file names from the above recipe to a function.
Something like

def handler(files):
for file in files:
im = Image.open(file)

Thanks


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Re: win32 How to make sure a file is completely written?

[justind]

On May 11, 10:03 am, Tim Golden m...@timgolden.me.uk wrote:
 justind wrote:
  Hello,

  I'm usinghttp://code.activestate.com/recipes/156178/to watch a
  folder in windows.

 Wow, that takes me back. There's a bit more info (and a different
 technique) here if you're interested:

  http://timgolden.me.uk/python/win32_how_do_i/watch_directory_for_chan...

 But to come back your question...

  It's working perfectly, but sometimes when I try to
  open the file immediately after receiving the event, it's not ready to
  be opened--if I try to open it with PIL I get IOError: cannot
  identify image file and if I try it with a text file, it's empty.
  This doesn't happen all the time, just occasionally. I think the
  problem is that the file isn't completely written because if I make
  the script sleep for a second, it works every time. But that doesn't
  seem very elegant or robust.

  What's the proper way to make sure the file is ready to be read?

 I don't believe there's an easy answer to this question. From the
 filesystem's point of view some process creates a file. So it
 tells you that a file has been created. The filesystem has no
 knowledge of when a file is complete. It could -- altho' it
 doesn't -- fire an event when a handle on that file is closed,
 but even that wouldn't tell you much: you'd have to be able
 to distinguish between the handle which is behind the data
 you're looking for and a handle from a virus scanner which is
 checking the new file for malware.

 To add difficulty, when Windows initiates a file copy (as opposed
 to creating a new file) it immediately creates the file at its
 full size and then copies data in. Which makes some sense as it
 should avoid some element of fragmentation etc. But it removes
 one of your options: noting the size of an original file and
 comparing it with the size of a copy.

 So I think you're out of luck and you're going to have to
 put a try-except loop in place.

 TJG

Thanks Guys.
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