Add period to evaluation monitor. (#6348)

tests/python/test_callback.py

@@ -34,10 +34,27 @@ class TestCallbacks(unittest.TestCase):
                   num_boost_round=rounds,
                   evals_result=evals_result,
                   verbose_eval=True)
-        print('evals_result:', evals_result)
         assert len(evals_result['Train']['error']) == rounds
         assert len(evals_result['Valid']['error']) == rounds
 
+        with tm.captured_output() as (out, err):
+            xgb.train({'objective': 'binary:logistic',
+                       'eval_metric': 'error'}, D_train,
+                      evals=[(D_train, 'Train'), (D_valid, 'Valid')],
+                      num_boost_round=rounds,
+                      evals_result=evals_result,
+                      verbose_eval=2)
+            output: str = out.getvalue().strip()
+
+        pos = 0
+        msg = 'Train-error'
+        for i in range(rounds // 2):
+            pos = output.find('Train-error', pos)
+            assert pos != -1
+            pos += len(msg)
+
+        assert output.find('Train-error', pos) == -1
+
     def test_early_stopping(self):
         D_train = xgb.DMatrix(self.X_train, self.y_train)
         D_valid = xgb.DMatrix(self.X_valid, self.y_valid)