added warning for early stopping in dart mode

python-package/lightgbm/callback.py

@@ -4,6 +4,7 @@
 from __future__ import absolute_import
 
 import collections
+import warnings
 from operator import gt, lt
 
 from .compat import range_
@@ -179,6 +180,9 @@ def early_stopping(stopping_rounds, verbose=True):
     cmp_op = []
 
     def _init(env):
+        if any((boost_alias in env.params
+                and env.params[boost_alias] == 'dart') for boost_alias in ('boosting', 'boosting_type', 'boost')):
+            warnings.warn('Early stopping is not available in dart mode')
         if not env.evaluation_result_list:
             raise ValueError('For early stopping, '
                              'at least one dataset and eval metric is required for evaluation')