Detect File Change Without Polling

Question

I'm trying to use a method within a Python program to detect whether a file on the file system has been modified. I know that I could have something run on an every-5-seconds to check the last modification date off of the system, but I was curious as to whether there's an easier method for doing this, without needing to require my program to check repeatedly.

Does anyone know of such a method?

What platform? Windows has a system service to register a callback when a directory or file changes. — PaulMcG, Apr 21 '11 at 01:17
Linux, but I'd prefer to make the program cross-platform compatible. — Thomas Ward, Apr 21 '11 at 01:19
If you're programming for Windows you might consider the .Net File System Watcher. http://www.ironpython.info/index.php/Watching_the_FileSystem — HK1, Apr 21 '11 at 01:23
@TobiasKienzler it is so-so, this question wants to watch a *single* file. — Antti Haapala -- Слава Україні, Apr 01 '16 at 08:30

score 83 · Answer 1 · edited Jan 08 '19 at 19:34

83

watchdog

Excellent cross platform library for watching directories.

From the website

Supported Platforms

Linux 2.6 (inotify)

Mac OS X (FSEvents, kqueue)

FreeBSD/BSD (kqueue)

Windows (ReadDirectoryChangesW with I/O completion ports; ReadDirectoryChangesW worker threads)

OS-independent (polling the disk for directory snapshots and comparing them periodically; slow and not recommended)

I've used it on a couple projects and it seems to work wonderfully.

edited Jan 08 '19 at 19:34

Facorazza

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answered Apr 21 '11 at 03:32

nemith

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The command-line tool is great for using in development. Thanks for the tip. – AgDude Dec 22 '11 at 13:39
From the example given on the watchdog website, it looks like it requires polling. – Zauberin Stardreamer Sep 15 '13 at 00:35
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@ZauberParacelsus: It does not. The example does `time.sleep(1)` to keep the example alive, but watchdog is actually asyncronous and will call event_handler in case of a file change event. – WhyNotHugo Apr 02 '14 at 14:19
It seems that *watchdog* only works for directories. Is there a way to watch a non-directory file for changes without poling (using the kernels inotify)? – Brutus Jun 19 '14 at 10:50

Aleksi Torhamo · Accepted Answer · 2016-09-03T20:24:12.577

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For linux, there is pyinotify.

From the homepage:

Pyinotify is a Python module for monitoring filesystems changes. Pyinotify relies on a Linux Kernel feature (merged in kernel 2.6.13) called inotify. inotify is an event-driven notifier, its notifications are exported from kernel space to user space through three system calls. pyinotify binds these system calls and provides an implementation on top of them offering a generic and abstract way to manipulate those functionalities.

Thus it is obviously not cross-platform and relies on a new enough kernel version. However, as far as I can see, requiring kernel support would be true about any non-polling mechanism.

edited Sep 03 '16 at 20:24

answered Apr 21 '11 at 01:20

Aleksi Torhamo

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Can pyinotify be used to detect the specific program that was used to modify a file? – Anderson Green Dec 29 '12 at 02:57
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Are the [objections against it](http://www.serpentine.com/blog/2008/01/04/why-you-should-not-use-pyinotify/comment-page-1/) still valid? – Brutus Jun 19 '14 at 10:30
this package exists no more – Ashoka Lella Sep 03 '16 at 18:52
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@AshokaLella: The package exists all right, but the project has moved from SF to github. I've updated the link. – Aleksi Torhamo Sep 03 '16 at 20:25

otherchirps · Answer 3 · 2017-04-20T00:44:46.487

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On windows there is:

watcher, which is a nice python port of the .NET FileSystemWatcher API.

Also there's (the one I wrote) dirwatch.

Both rely on the windows ReadDirectoryChangesW function. Though for real work, I'd use watcher (proper C extension, good API, python 2 & 3 support).

Mine is mostly an experiment calling the relevant APIs on windows, so it's only interesting if you want an example of calling these things from python.

edited Apr 20 '17 at 00:44

answered Apr 21 '11 at 02:28

otherchirps

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watcher seems to have been abandoned for a while and lacks proper documentation. – chiffa Sep 01 '15 at 18:54
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Yep, the scenery has changed since 2011... Looks like watchdog is the best bet these days. – otherchirps Sep 01 '15 at 22:56

score 5 · Answer 4 · answered Apr 21 '11 at 01:32

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You should also see inotifyx which is very similar to the previously mentioned pyinotify, but is said to have an API which changes less.

answered Apr 21 '11 at 01:32

AJ00200

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Detect File Change Without Polling

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