This is not so straightforward, the subprocesses needs to be reimplemented without watchers before removing it. That has to be done carefully, the code is fragile here with lots of edge cases to deal with.

the subprocesses needs to be reimplemented without watchers before removing it

I want to get a small advice about will it be enough to keep only one, standart ChildWatcher?

@SweetyAngel:
FWIW Previous discussions are at gh-94597 and gh-82772. IIRC the intention was to remove all APIs for getting/setting child watchers, but to leave one or more implementations as an internal detail.

I want to get a small advice about will it be enough to keep only one, standart ChildWatcher?

I think that implementation could either be ThreadedChildWatcher or it could be a choice between ThreadedChildWatcher and PidfdChildWatcher depending on the platform (the latter on Linux, the former on all other UNIX systems). The current code uses that:

    def _init_watcher(self):
        with events._lock:
            if self._watcher is None:  # pragma: no branch
                if can_use_pidfd():
                    self._watcher = PidfdChildWatcher()
                else:
                    self._watcher = ThreadedChildWatcher()

Perhaps we can keep this approach for now, and just remove the deprecated watcher classes? (BaseChildWatcher and its subclasses, SafeChildWatcher, FastChildWatcher and MultiLoopChildWatcher.)

@SweetyAngel Would you like to continue work on this as Guido suggested? If not I'll take over and do it.

I ve already done it but i need to move it on new branch. New PR will appear in a few days

I forgot actually I already have code for this, in a old branch, I'll resolve it and get it done faster. it will also include rewrite on child watchers.

I am creating a new issue for this with much smaller items to work with.