Adding callbacks after instantiating FileSystemEventHandler doesn't fire the callback.

[unique1o1]

OS: Pop!_OS 20.04 LTS
Python version: 3.9.5
Watchdog version: 2.1.4

Adding callbacks (on_deleted, on_created...) after instantiating FileSystemEventHandler class(and its subclasses) will not fire the respective callback when a event is trigger

For example:
This Works

class myEventHandler(RegexMatchingEventHandler):
    def on_created(self,event):
        print(f" created!")

    def on_deleted(self,event):
        print(f"deleted")

    def on_modified(self,event):
        print(f"modified")

    def on_moved(self,event):
        print(f"moved")
my_event_handler = myEventHandler(
    [".*"], None, False, False
)
path = "."
my_observer = Observer()
my_observer.schedule(my_event_handler, path, recursive=True)
my_observer.start()
try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    my_observer.stop()
    my_observer.join()

This doesn't

def on_created(self,event):
    print(f" created!")
def on_deleted(self,event):
    print(f"deleted")
def on_modified(self,event):
    print(f"modified")
def on_moved(self,event):
    print(f"moved")

path = "."
my_event_handler = RegexMatchingEventHandler(
    [".*"], None, False, False
)
my_event_handler.on_created = on_created
my_event_handler.on_deleted = on_deleted
my_event_handler.on_modified = on_modified
my_event_handler.on_moved = on_moved

my_observer = Observer()
my_observer.schedule(my_event_handler, path, recursive=True)
my_observer.start()
try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    my_observer.stop()
    my_observer.join()

changes from the PR #814 seems to be the reason for the issue.

EDIT: Sorry if this is not the preferred way of doing things. I just started using watchdog and many articles on the web used this approach and mine was not working, which got me scratching my head hard. I tried the above code on the older version i.e (2.1.3) and it worked.

[BoboTiG]

Thanks, it is a duplicate of #830. I am closing that one and focus on #830.