[python-package] fix mypy errors related to eval result parsing in callbacks

[jameslamb]

Contributes to #3756.
Contributes to #3867.

Resolves the following errors from mypy:

python-package/lightgbm/callback.py:128: error: Item "None" of "Optional[List[Tuple[str, str, float, bool]]]" has no attribute "__iter__" (not iterable)  [union-attr]
python-package/lightgbm/callback.py:143: error: Item "None" of "Optional[List[Tuple[str, str, float, bool]]]" has no attribute "__iter__" (not iterable)  [union-attr]
python-package/lightgbm/callback.py:283: error: Argument 1 to "len" has incompatible type "Optional[List[Tuple[str, str, float, bool]]]"; expected "Sized"  [arg-type]
python-package/lightgbm/callback.py:285: error: Value of type "Optional[List[Tuple[str, str, float, bool]]]" is not indexable  [index]
python-package/lightgbm/callback.py:286: error: Value of type "Optional[List[Tuple[str, str, float, bool]]]" is not indexable  [index]
python-package/lightgbm/callback.py:362: error: Argument 1 to "len" has incompatible type "Optional[List[Tuple[str, str, float, bool]]]"; expected "Sized"  [arg-type]
python-package/lightgbm/callback.py:363: error: Value of type "Optional[List[Tuple[str, str, float, bool]]]" is not indexable  [index]
python-package/lightgbm/callback.py:368: error: Argument 1 to "append" of "list" has incompatible type "Optional[List[Tuple[str, str, float, bool]]]"; expected "Union[List[Tuple[str, str, float, bool]], List[Tuple[str, str, float, bool, float]]]"  [arg-type]
python-package/lightgbm/callback.py:370: error: Incompatible types in assignment (expression has type "Optional[List[Tuple[str, str, float, bool]]]", target has type "Union[List[Tuple[str, str, float, bool]], List[Tuple[str, str, float, bool, float]]]")  [assignment]
python-package/lightgbm/callback.py:372: error: Value of type "Optional[List[Tuple[str, str, float, bool]]]" is not indexable  [index]
python-package/lightgbm/callback.py:375: error: Value of type "Optional[List[Tuple[str, str, float, bool]]]" is not indexable  [index]

These are all mypy rightly identifying places in the callbacks code where CallbackEnv.evaluation_result_list is used unconditionally as if it contains a list...

LightGBM/python-package/lightgbm/callback.py

Lines 126 to 128 in 921479b

	def _init(self, env: CallbackEnv) -> None:
	self.eval_result.clear()
	for item in env.evaluation_result_list:

... even though it's possible for it to be None.

LightGBM/python-package/lightgbm/engine.py

Lines 269 to 274 in 921479b

	cb(callback.CallbackEnv(model=booster,
	params=params,
	iteration=i,
	begin_iteration=init_iteration,
	end_iteration=init_iteration + num_boost_round,
	evaluation_result_list=None))

This PR proposes defensively raising an informative error if env.evaluation_result_list is still None when any of those codepaths are reached. These should never be encountered by user code, but if they are then they're replacing cryptic errors like TypeError: 'NoneType' object is not subscriptable with something clearer.

Why not just enforce that CallbackEnv.evaluation_result_list is always a list and never None?

I'm afraid that that would break other integrations with lightgbm.

evaluation_result_list has been nullable since the earliest days of the Python package, 7+ years ago: #97. I think taking on these errors messages is preferable to risking that.

Especially since I've learned tonight that other projects are relying on the callbacks mechanism to integration with lightgbmm, like:

• FLAML (code link)
• autogluon (code link)
• wandb (code link)

[github-actions]

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