The code is *not* self-explanatory! However, I will give an explanation and you are welcome to include it in a future revision.
cevent.py defines a lot of subroutines that are only called, they don't actually do anything, they are just placeholders. If one of those subroutines is called (when one of those events happens) and that subroutine is not present, then the program will crash It is better that an event is ignored than a program crash.
Any python program that imports cevent and then defines a method with the same name as a method in cenvent.py will use the method it defined. This is called "overriding", as the subroutine for the event that your program used will override the the subroutine already defined in cevent.py.
If you are testing your software and you want to make sure that every event is handled or there is an error, then replace "pass" in cevent.py with "raise NotImplementedError(SOME EXPLANATION STRING HERE)". That will raise the NotImplementedError exception and create a nice traceback that you can follow to find out what the error was.