Allow scheduling watches without an initial event handler (fixes #1039)

[Prosperibe12]

🚀 Description

Implements handler-less watches: Observer.schedule(None, path, …) (or schedule(noop_event_handler, path, …)) registers the watch without an initial handler; handlers are added later with add_handler_for_watch. Fixes unschedule when no handlers exist.

💁 Why is this change needed?

Several call sites need one shared watch on a path while attaching handlers from different subroutines. Previously schedule always registered an initial handler, and passing None stored None in the handler set, which could break dispatch. This implements the approach discussed on the issue: optional None / no-op handler at schedule time, then add_handler_for_watch for real handlers.

📎 Issue (#1039)

✨ Type of change

• New feature (non-breaking change which adds functionality)

📋 New feature implementation

• BaseObserver.schedule: skip _add_handler_for_watch when event_handler is None; @overload for FileSystemEventHandler vs None.
• watchdog.events: NoOpEventHandler, noop_event_handler.
• BaseObserver.unschedule: use _handlers.pop(watch, None).
• FSEventsObserver.schedule: same overloads / FileSystemEventHandler | None as the base class.

from watchdog.events import FileSystemEventHandler, FileModifiedEvent
from watchdog.observers import Observer


class Handler0(FileSystemEventHandler):
    def on_modified(self, event: FileModifiedEvent) -> None:
        print("handler0", event.src_path)


class Handler1(FileSystemEventHandler):
    def on_modified(self, event: FileModifiedEvent) -> None:
        print("handler1", event.src_path)


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 = Observer()
watch = observer.schedule(None, "/path/to/dir", recursive=True)
# Typing-friendly alternative:
# from watchdog.events import noop_event_handler
# watch = observer.schedule(noop_event_handler, "/path/to/dir", recursive=True)

do_something0(observer, watch)
do_something1(observer, watch)
observer.start()

[Prosperibe12]

@BoboTiG I will appreaciate a review on this pr

[Prosperibe12]

@BoboTiG commenting again, maybe you have missed the previous message