How do I watch a file for changes?

Question

I have a log file being written by another process which I want to watch for changes. Each time a change occurs I'd like to read the new data in to do some processing on it.

What's the best way to do this? I was hoping there'd be some sort of hook from the PyWin32 library. I've found the win32file.FindNextChangeNotification function but have no idea how to ask it to watch a specific file.

If anyone's done anything like this I'd be really grateful to hear how...

[Edit] I should have mentioned that I was after a solution that doesn't require polling.

[Edit] Curses! It seems this doesn't work over a mapped network drive. I'm guessing windows doesn't 'hear' any updates to the file the way it does on a local disk.

Answer 1

Did you try using Watchdog?

Python API library and shell utilities to monitor file system events.

Directory monitoring made easy with

• A cross-platform API.
• A shell tool to run commands in response to directory changes.

Get started quickly with a simple example in Quickstart...

Answer 2

If polling is good enough for you, I'd just watch if the "modified time" file stat changes. To read it:

os.stat(filename).st_mtime 

(Also note that the Windows native change event solution does not work in all circumstances, e.g. on network drives.)

import os  class Monkey(object):     def __init__(self):         self._cached_stamp = 0         self.filename = '/path/to/file'      def ook(self):         stamp = os.stat(self.filename).st_mtime         if stamp != self._cached_stamp:             self._cached_stamp = stamp             # File has changed, so do something...