The name of this function is confusing, since it doesn't get the default codebase - there is no "default codebase".  It doesn't even apply any defaults (since it indexes the dict assuming the key exists).  How about `getCodebaseDict`?

Also, we're not using docstrings anymore, as they tend to lead people to look at documentation generated from them, which leads those people to believe that everything with a docstring is part of the API.

In the nine branch, there's a `master/docs/developer/schedulers.rst` with info on how to write a scheduler, and I think it would make sense to put this information there.  However, that's not the branch you're developing against.

So: please convert this to a comment.