Thank you for the quick reply!
The SegmentEditorEffects extention had been installed in the software.Even if I downloaded the SegmentEditorHollow.py,add its path into the additional module and restart 3D Slicer,the button of the hollow have not shown in the Effects file as below.What is the reason about this?Otherwise,there is a file named “init.py”,what is the meaning about is?
If you installed SegmentEditorExtraEffects extension for Slicer-4.8.1 before, then you need to update it (or uninstall and install it again) to get the new effect.
For manual install, it is not enough to copy a single file but the entire content of SegmentEditorHollow folder, including subdirectories. The folder containing SegmentEditorHollow.py must be added to Additional module paths.