I confirm the fix.

It is clear that the separation between BaseChildWatcher and its subclasses is far from ideal. The first implementation was clean, but as the patch evolved interactions got complex to the point that BaseChildWatcher should not be considered as an API but rather as implementation details for the two other classes (eg. .add_child_handler() is implemented in subclasses whereas .remove_child_handler() is in the base class). At least it should be renamed as _BaseChildWatcher to make that clear.

For set_loop, another possible name is 'attach_loop' (like in the doc string actually)