How to add a custom preprocessor?

[rinvii]

Operating system

nbgrader --version

0.9.3

jupyterhub --version (if used with JupyterHub)

4.1.6

jupyter notebook --version

7.2.1

Feature Request

Is there a reliable way to create custom preprocessors? There is mention of custom preprocessors here, but it just leads to esoteric documentation.

I tried to create a custom preprocessor in the same directory as nbgrader_config.py and included it in c.GenerateAssignment.preprocessors. However, that didn't work.

Because of this, I had to go into the nbgrader source code itself, write my custom preprocessor inside /nbgrader/preprocessors, adjust /nbgrader/preprocessors/__init__.py, and also adjust /nbgrader/converters/generate_assignment.py. All of this just to include my own preprocessor.

There must be a more reliable way to do this instead of going directly to the source, no? If there is, and if I'm missing something obvious in the docs, please let me know if there are any examples. Thank you.