[python][R-package] removed duplicated code from language wrappers

[StrikerRUS]

Removed checks from language wrappers that are handled at cpp side.

LightGBM/src/c_api.cpp

Lines 176 to 187 in bbc45fe

	void ResetConfig(const char* parameters) {
	std::lock_guard<std::mutex> lock(mutex_);
	auto param = Config::Str2Map(parameters);
	if (param.count("num_class")) {
	Log::Fatal("Cannot change num_class during training");
	}
	if (param.count("boosting")) {
	Log::Fatal("Cannot change boosting during training");
	}
	if (param.count("metric")) {
	Log::Fatal("Cannot change metric during training");
	}

[StrikerRUS]

@guolinke According to this commit, seems that metric cannot be reset anymore, right?
714c673
So, this code is outdated too.

LightGBM/python-package/lightgbm/basic.py

Lines 1916 to 1917 in bbc45fe

	if any(metric_alias in params for metric_alias in _ConfigAliases.get("metric")):
	self.__need_reload_eval_info = True

[jameslamb]

Looks great! I'm glad we can push more of this logic into the cpp side. Thank you!

[StrikerRUS]

@guolinke Can you please confirm this #2606 (comment)?

[guolinke]

@StrikerRUS it seems you are right. but I don't remember why __need_reload_eval_info is there...

[StrikerRUS]

@guolinke

it seems you are right. but I don't remember why __need_reload_eval_info is there...

Maybe in some point in the past it was possible to reset metric param? __need_reload_eval_info is used only once here:

LightGBM/python-package/lightgbm/basic.py

Lines 2691 to 2694 in f3afe98

	def __get_eval_info(self):
	"""Get inner evaluation count and names."""
	if self.__need_reload_eval_info:
	self.__need_reload_eval_info = False

Anyway, as at present resetting metric leads to fatal error, keeping that code seems to be unreasonable.

[jameslamb]

I re-reviewed since some time has passed since the PR was opened. R side still looks good to me!

[StrikerRUS]

Thanks @jameslamb !

I'm about to merge this PR, but want to make sure that @guolinke hasn't accidentally remembered why self.__need_reload_eval_info = True was here.

[guolinke]

@StrikerRUS I think you can remove the __need_reload_eval_info