Watchdog

Introduction

The Watchdog Extension includes the WatchdogManager, enabling applications to easily monitor, and react to, changes in filesystem paths based on the filesystem events monitoring library Watchdog.

On application startup, the Watchdog Observer is automatically started and then upon application close, the observer thread is properly stopped and joined with the parent process before exit.

API References:

• Cement Watchdog Extension

• Watchdog Library

Requirements

• Watchdog

Cement 3.0.8+:

pip install cement[watchdog]

Applications using Cement <3.0.8 should continue to include watchdog in their dependencies.

Platform Support

• Unix/Linux

• macOS

• Windows

Configuration

Application Configuration Settings

This extension does not support any application level configuration settings.

Application Meta Options

This extension honors the following application meta options:

Option	Description
watchdog_paths	A list of tuples that are passed directly as arguments to WatchdogManager.add() (a shortcut equivalent to app.watchdog.add().

Hooks

This extension defines the following hooks:

watchdog_pre_start

Run first when App.watchdog.start() is called. The application object is passed as an argument. Nothing is expected in return.

watchdog_post_start

Run last when App.watchdog.start() is called. The application object is passed as an argument. Nothing is expected in return.

watchdog_pre_stop

Run first when App.watchdog.stop() is called. The application object is passed as an argument. Nothing is expected in return.

watchdog_post_stop

Run last when App.watchdog.stop() is called. The application object is passed as an argument. Nothing is expected in return.

watchdog_pre_join

Run first when App.watchdog.join() is called. The application object is passed as an argument. Nothing is expected in return.

watchdog_post_join

Run last when App.watchdog.join() is called. The application object is passed as an argument. Nothing is expected in return.

Usage

The following example uses the default WatchdogEventHandler that by default only logs all events to debug:

Example: Using Watchdog Extension

from time import sleep
from cement import App, CaughtSignal
from cement.ext.ext_watchdog import WatchdogEventHandler

class MyApp(App):
    class Meta:
        label = 'myapp'
        extensions = ['watchdog']
        watchdog_paths = [
            ('./tmp/', WatchdogEventHandler),
        ]

with MyApp() as app:
    app.run()
    try:
        while True:
            sleep(1)
    except CaughtSignal as e:
        print(e)

In the above example, nothing is printed to console. However you will see something like the following via debug logging:

$ python myapp.py --debug 2>&1 | grep -i watchdog
cement.core.extension : loading the 'cement.ext.ext_watchdog' framework extension
cement.core.hook : defining hook 'watchdog_pre_start'
cement.core.hook : defining hook 'watchdog_post_start'
cement.core.hook : defining hook 'watchdog_pre_stop'
cement.core.hook : defining hook 'watchdog_post_stop'
cement.core.hook : defining hook 'watchdog_pre_join'
cement.core.hook : defining hook 'watchdog_post_join'
cement.core.hook : registering hook 'watchdog_extend_app' from cement.ext.ext_watchdog into hooks['post_setup']
cement.core.hook : registering hook 'watchdog_add_paths' from cement.ext.ext_watchdog into hooks['post_setup']
cement.core.hook : registering hook 'watchdog_start' from cement.ext.ext_watchdog into hooks['pre_run']
cement.core.hook : registering hook 'watchdog_cleanup' from cement.ext.ext_watchdog into hooks['pre_close']
cement.core.hook : running hook 'post_setup' (<function watchdog_extend_app at 0x103c991e0>) from cement.ext.ext_watchdog
cement.core.foundation : extending appication with '.watchdog' (<cement.ext.ext_watchdog.WatchdogManager object at 0x103f83ef0>)
cement.core.hook : running hook 'post_setup' (<function watchdog_add_paths at 0x103ddd6a8>) from cement.ext.ext_watchdog
cement.ext.ext_watchdog : adding path /path/to/tmp with event handler <class 'cement.ext.ext_watchdog.WatchdogEventHandler'>
cement.core.hook : running hook 'pre_run' (<function watchdog_start at 0x103ddd598>) from cement.ext.ext_watchdog
cement.ext.ext_watchdog : starting watchdog observer
myapp : Watchdog Event: <FileDeletedEvent: src_path='/path/to/tmp/test2'>
myapp : Watchdog Event: <FileDeletedEvent: src_path='/path/to/tmp/test4'>
myapp : Watchdog Event: <FileDeletedEvent: src_path='/path/to/tmp/test3'>
myapp : Watchdog Event: <FileDeletedEvent: src_path='/path/to/tmp/dir1/test'>
myapp : Watchdog Event: <FileDeletedEvent: src_path='/path/to/tmp/test'>
myapp : Watchdog Event: <DirDeletedEvent: src_path='/path/to/tmp/dir1'>
myapp : Watchdog Event: <DirModifiedEvent: src_path='/path/to/tmp'>
myapp : Watchdog Event: <DirModifiedEvent: src_path='/path/to/tmp'>
myapp : Watchdog Event: <DirCreatedEvent: src_path='/path/to/tmp/dir1'>
myapp : Watchdog Event: <FileCreatedEvent: src_path='/path/to/tmp/dir1/test.file'>
myapp : Watchdog Event: <DirModifiedEvent: src_path='/path/to/tmp/dir1'>
cement.core.hook : running hook 'pre_close' (<function watchdog_cleanup at 0x10e930620>) from cement.ext.ext_watchdog
cement.ext.ext_watchdog : stopping watchdog observer
cement.ext.ext_watchdog : joining watchdog observer
cement.core.foundation : closing the myapp application

To expand on the above example, we can add our own event handlers:

Example: Adding Watchdog Event Handlers

myapp.py

from time import sleep
from cement import App, CaughtSignal
from cement.ext.ext_watchdog import WatchdogEventHandler

class MyEventHandler(WatchdogEventHandler):
    def on_any_event(self, event):
        # do something with the ``event`` object
        print("The modified path was: %s" % event.src_path)

class MyApp(App):
    class Meta:
        label = 'myapp'
        extensions = ['watchdog']
        watchdog_paths = [
            ('./tmp/', MyEventHandler),
        ]

with MyApp() as app:
    app.run()

    try:
        while True:
            sleep(1)
    except CaughtSignal as e:
        print(e)

cli

$ python myapp.py
The modified path was: /path/to/tmp/test.file

Note that the WatchdogEventHandler could be replaced with any other event handler classe (i.e. those available from watchdog directly). However to play nicely with Cement, we sub-class them first in order to pass in our application object.

Example: Creating a Watchdog Event Handler

from watchdog.events import FileSystemEventHandler

class MyEventHandler(FileSystemEventHandler):
    def __init__(self, app, *args, **kw):
        super(MyEventHandler, self).__init__(*args, **kw)
        self.app = app

For full usage of Watchdog event handlers, refer to the Watchdog API Documentation.