fix #1318

docs/Parameters.rst

@@ -576,9 +576,9 @@ Objective Parameters
 Metric Parameters
 -----------------
 
--  ``metric``, default=\ ``None``, type=multi-enum
+-  ``metric``, default=\ ``""``, type=multi-enum
 
-   -  if ``None``, metric corresponding to specified application will be used
+   -  if ``""``(empty string or not specific), metric corresponding to specified application will be used
 
    -  ``l1``, absolute loss, alias=\ ``mean_absolute_error``, ``mae``, ``regression_l1``
 

src/io/config.cpp

@@ -91,7 +91,7 @@ void GetMetricType(const std::unordered_map<std::string, std::string>& params, s
     metric_types->shrink_to_fit();
   }
   // add names of objective function if not providing metric
-  if (metric_types->empty()) {
+  if (metric_types->empty() && value.size() == 0) {
     if (ConfigBase::GetString(params, "objective", &value)) {
       std::transform(value.begin(), value.end(), value.begin(), Common::tolower);
       metric_types->push_back(value);