Export processors extension system

[jph00]

A simple, clean extension system for export processors — functions that transform code cells during nbdev_export.

Changes

New extension system

• export_procs config in [tool.nbdev] accepts a list of module:callable strings
• Each processor is a simple function: def my_proc(cell) that modifies cell.source in place
• No entry points or magic — just explicit imports

Example config:

[tool.nbdev]
export_procs = ["nbdev.scrubmagics:scrub_magics"]

Removed from core

• black_format — moved to separate nbdev-black package
• scrub_magics — moved to nbdev/scrubmagics.py (still ships with nbdev, but opt-in)
• optional_procs() — no longer needed

Other changes

• Added import_obj(s) to config.py for importing module:obj strings
• Renamed procs → doc_procs in config for clarity (doc processors vs export processors)
• Config defaults for doc_procs and export_procs are now lists [] instead of empty strings

Documentation

• Updated nbs/tutorials/extensions.ipynb with:
  • Built-in scrub_magics processor docs
  • New section on writing export processors
  • Comparison of doc processors vs export processors

Breaking Changes

• black_format no longer in core — install nbdev-black and add ["nbdevblack:black_format"] to export_procs
• procs config renamed to doc_procs
• black_formatting config option removed (was deprecated)