[Feature] Allow adding ObservedWatch without handlers

[toluschr]

Allow adding ObservedWatch without handlers

Why is this helpful?

In certain cases, a programmer may call multiple subroutines that shall watch the same file. With the current implementation, this is cumbersome to do. Allowing the programmer to add an empty ObservedWatch could allow for a much simplified implementation.

What should be implemented?

Either: Add a function Observer.watch(self, filename, recursive=False, event_filter=None) that creates a watch without handlers.

def do_something0(observer, watch):
    observer.add_handler_for_watch(handler0, watch)

def do_something1(observer, watch):
    observer.add_handler_for_watch(handler1, watch)

observer = watchdog.observers.Observer()
watch = observer.watch("file") # proposal
do_something0(observer, watch) # do_something adds handlers for "file"
do_something1(observer, watch) # do_something adds handlers for "file"
observer.start()

Or: Allow passing a event_handler=None to Observer.schedule to create an ObservedWatch without a handler.

def do_something0(observer, watch):
    observer.add_handler_for_watch(handler0, watch)

def do_something1(observer, watch):
    observer.add_handler_for_watch(handler1, watch)

observer = watchdog.observers.Observer()
watch = observer.schedule(None, "file") # proposal
do_something0(observer, watch) # do_something adds handlers for "file"
do_something1(observer, watch) # do_something adds handlers for "file"
observer.start()

Optionally: Store a reference to the observer in ObservedWatch to simplify this API

def do_something0(watch):
    watch.add_handler(handler0)

def do_something1(watch):
    watch.add_handler(handler1)

observer = watchdog.observers.Observer()
watch = observer.watch("file")
do_something0(watch) # do_something adds handlers for "file"
do_something1(watch) # do_something adds handlers for "file"
observer.start()