Description

While working on #3756 and #3867, as well as a variety of early-stopping related things (like #6424), I've found the heavy use of positional indexing and tuple unpacking in early stopping and related code make it really difficult to understand and modify.

For example:

https://github.com/microsoft/LightGBM/blob/53e0ddf7cd6eb281e3bec6273b19ff541c69bfa6/python-package/lightgbm/callback.py#L167-L176

and:

https://github.com/microsoft/LightGBM/blob/53e0ddf7cd6eb281e3bec6273b19ff541c69bfa6/python-package/lightgbm/callback.py#L410-L412

I'm opening this issue to track some work I'd like to do to simplify that.

Benefits of this work

Reduces the effort required to finish these:

• [python-package] type hints in python package #3756
• [ci] warnings from mypy #3867

And to add finer-grained control over early stopping and validation, e.g.:

• [WIP] Add chosen metric argument to clarify early stopping behaviour #6424 (comment)

Approach

I'm planning a series of PRs with the following types of changes:

• unpacking tuples into named variables and using those named variables
• introducing collections.namedtuple (docs) where appropriate to allow for named-attribute access while preserving backwards compatibility with the significant amount of custom code in the world relying on lightgbm expecting tuples for this like custom metrics:

https://github.com/microsoft/LightGBM/blob/53e0ddf7cd6eb281e3bec6273b19ff541c69bfa6/python-package/lightgbm/engine.py#L135-L138

Notes

Looking at the current state of this code, it's important to remember that when the lightgbm Python package was first introduced 8+ years ago:

• it supported Python 2.7
• dataclasses was not in the standard library yet (that came in Python 3.7, first release in June 2018)