[python] fix dangerous default for eval_at in LGBMRanker (#3377)

* [python] fix dangerous default for eval_at in LGBMRanker

* use a tuple

* five

* Update python-package/lightgbm/sklearn.py
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

python-package/lightgbm/sklearn.py

@@ -945,7 +945,7 @@ class LGBMRanker(LGBMModel):
             sample_weight=None, init_score=None, group=None,
             eval_set=None, eval_names=None, eval_sample_weight=None,
             eval_init_score=None, eval_group=None, eval_metric=None,
-            eval_at=[1, 2, 3, 4, 5], early_stopping_rounds=None, verbose=True,
+            eval_at=(1, 2, 3, 4, 5), early_stopping_rounds=None, verbose=True,
             feature_name='auto', categorical_feature='auto',
             callbacks=None, init_model=None):
         """Docstring is inherited from the LGBMModel."""
@@ -984,6 +984,6 @@ class LGBMRanker(LGBMModel):
     _base_doc = fit.__doc__
     _before_early_stop, _early_stop, _after_early_stop = _base_doc.partition('early_stopping_rounds :')
     fit.__doc__ = (_before_early_stop
-                   + 'eval_at : list of int, optional (default=[1, 2, 3, 4, 5])\n'
+                   + 'eval_at : iterable of int, optional (default=(1, 2, 3, 4, 5))\n'
                    + ' ' * 12 + 'The evaluation positions of the specified metric.\n'
                    + ' ' * 8 + _early_stop + _after_early_stop)