Fix handling of print period in EvaluationMonitor (dmlc#6499)

Co-authored-by: Kirill Shvets <kirill.shvets@intel.com>

python-package/xgboost/callback.py

@@ -622,7 +622,7 @@ def after_iteration(self, model, epoch, evals_log):
                     msg += self._fmt_metric(data, metric_name, score, stdv)
             msg += '\n'
 
-            if (epoch % self.period) != 0 or self.period == 1:
+            if (epoch % self.period) == 0 or self.period == 1:
                 rabit.tracker_print(msg)
                 self._latest = None
             else:

tests/python/test_callback.py

@@ -33,15 +33,18 @@ def run_evaluation_monitor(self, D_train, D_valid, rounds, verbose_eval):
                       verbose_eval=verbose_eval)
             output: str = out.getvalue().strip()
 
-        pos = 0
-        msg = 'Train-error'
-        for i in range(rounds // int(verbose_eval)):
-            pos = output.find('Train-error', pos)
-            assert pos != -1
-            pos += len(msg)
-
-        assert output.find('Train-error', pos) == -1
-
+        if int(verbose_eval) == 1:
+            # Should print each iteration info
+            assert len(output.split('\n')) == rounds
+        elif int(verbose_eval) > rounds:
+            # Should print first and latest iteration info
+            assert len(output.split('\n')) == 2
+        else:
+            # Should print info by each period additionaly to first and latest iteration
+            num_periods = rounds // int(verbose_eval)
+            # Extra information is required for latest iteration
+            is_extra_info_required = num_periods * int(verbose_eval) < (rounds - 1) 
+            assert len(output.split('\n')) == 1 + num_periods + int(is_extra_info_required)
 
     def test_evaluation_monitor(self):
         D_train = xgb.DMatrix(self.X_train, self.y_train)
@@ -57,8 +60,10 @@ def test_evaluation_monitor(self):
         assert len(evals_result['Train']['error']) == rounds
         assert len(evals_result['Valid']['error']) == rounds
 
-        self.run_evaluation_monitor(D_train, D_valid, rounds, 2)
         self.run_evaluation_monitor(D_train, D_valid, rounds, True)
+        self.run_evaluation_monitor(D_train, D_valid, rounds, 2)
+        self.run_evaluation_monitor(D_train, D_valid, rounds, 4)
+        self.run_evaluation_monitor(D_train, D_valid, rounds, rounds + 1)        
 
     def test_early_stopping(self):
         D_train = xgb.DMatrix(self.X_train, self.y_train)