https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_styleguide.html#class-declaration
Make it as explicit as possible. Don't worry about being verbose. :)
Fixed in https://hub.darcs.net/ThomasLocke/godot-playground/patch/48e770f7931911ad6474caeb02f1bf9558d44598