[python-package] enforce keyword-only args in more internal functions

[jameslamb]

Contributes to #3756

Enforces keyword-only arguments in more internal functions, to make the Python codebase a bit stricter and easier to follow.

[StrikerRUS]

Thanks! Just one minor question below

[StrikerRUS]

Just curious, why not after self?

[jameslamb]

I was thinking that a binary "op" type of function should always begin with 2 positional arguments, just something I'm used to from other languages.

But I see this is only called by lightgbm's own code (not passed through anything from functools or something) and is called like any other normal function:

LightGBM/python-package/lightgbm/callback.py

Line 416 in 33e93d6

if first_time_updating_best_score_list or self.cmp_op[i](metric_value, self.best_score[i]):

Seeing that, I think keyword arguments there would improve clarity. I'll make that change (and for _lt_delta).

[jameslamb]

Updated in 2e48184

I had to add a # type: ignore[call-arg] comment. I think mypy is confused by the use of partial() here:

LightGBM/python-package/lightgbm/callback.py

Line 386 in 33e93d6

self.cmp_op.append(partial(self._gt_delta, delta=delta))

python-package/lightgbm/callback.py:416: error: Unexpected keyword argument "curr_score"  [call-arg]
python-package/lightgbm/callback.py:416: error: Unexpected keyword argument "best_score"  [call-arg]

[StrikerRUS]

Thanks for making calls of these functions even more clear.

[borchero]

Nice!