The problem is, what do I do if I need a module's functionality but don't want to rewrite it?

Two possibilities spring to mind.

1. Simply write a module which uses the other. This is probably the appropriate way to do it if you are adding functionality to a module anyway. Subclassing, I think, is best when you want to change the functionality, particularly by making it more specific.

2. A tried and true method that doesn't get enough respect these days is to create a copy of the module and modify it. Forking a module's development is a perfectly fine form of code reuse which, like any other, has its advantages and its disadvantages.

-sauoq
"My two cents aren't worth a dime.";

I am one of those few who have to watch for false hubris more than false laziness, so this is what I'd want to do:

Patch the module to clean up the internals, and add hooks - whether that be subclassability, slots for user-supplied callbacks or whatever else may seem appropriate -, and of course submit the patch back to the author. Then I'd use the well-defined interface I just created to add my own, case-specific functionality inside my application.

Hopefully the author will either accept the patch or be inspired - and so whoever next needs something like me will also benefit.

Makeshifts last the longest.