Shouldn't call close on already closed fd...

[giannitedesco]

If WatchManager.close() is called twice then bad things can happen :)
Also if WatchManager.close() is called then Notifier.stop() the same thing can happen.
Basically, there needs to be a well defined lifetime/ownership for the fd otherwise we can start randomly closing the fd's belonging to other objects which don't expect it.

[giannitedesco]

Maybe context managers would be nice too, that'd make it clear.