[python-package] Keep constants in sync with C++ library

[cyfdecyf]

Relevant only when #4089 is merged.

Summary

Find a way to keep constants in Python package in sync with C++ library.

This is needed in order to keep python package behaves the same as the cmdline version.

Motivation

#4089 handles data sampling in Python. It defines 2 constant variables ZERO_THRESHOLD and DEFAULT_BIN_CONSTRUCT_SAMPLE_CNT.

Currently they are set to the same value as in C++. If the default value changs in C++ side, we have to manually change them in Python side.

Description

I suggest we expose relevant constants in C API and use them in other language packages.

We can define a map which contains constant name and their value. Then add a C API to get those values, e.g.

const char* LGBM_GetConstant(const char* key);

References

Relevant code in #4089

Constants defined in pyhon package.
https://github.com/cyfdecyf/LightGBM/blob/f86d2db89d89810ddac0bc842af8d392a47edc85/python-package/lightgbm/basic.py#L21-L22

Usage:
https://github.com/cyfdecyf/LightGBM/blob/f86d2db89d89810ddac0bc842af8d392a47edc85/python-package/lightgbm/basic.py#L1556
https://github.com/cyfdecyf/LightGBM/blob/f86d2db89d89810ddac0bc842af8d392a47edc85/python-package/lightgbm/basic.py#L1217
https://github.com/cyfdecyf/LightGBM/blob/f86d2db89d89810ddac0bc842af8d392a47edc85/python-package/lightgbm/basic.py#L1580

[shiyu1994]

@cyfdecyf Thank you! Very good idea. And I believe it can be useful in the future.

[jameslamb]

Thank you for this excellent write-up! I've added this to #2302 with other feature requests, so I'll close this issue for now.

Anyone interested in contributing this is welcome to do so. Leave a comment below and the discussion can be re-opened.