title: XGBoostError: [09:48:50] /usr/local/xgboost/src/metric/metric.cc:21: Unknown metric function auc
body: <p>I'm using xgboost to predict, and my main code is the following:</p>
  
  <pre><code>params={
      'booster':'gbtree',
      'object':'binary:logistic',
      'early_stopping_rounds':100,
      'eval_metric':'auc',
      'gamma':0.1,
      'max_depth':8,
      'lambda':550,
      'subsample':0.7,
      'colsample_bytree':0.2,
      'min_child_weight':3,
      'eta':0.02,
      'seed':2,
      'min_child_weight':0.5,
      'scale_pos_weight':ratio,
      'max_delta_step':5, 
      'nthread':7
  }
  
  watch_list = [(dval,'val'),(dtrain,'train')]
  xgboost_model = xgb.train(params,dtrain,num_boost_round=500, evals = watch_list)
  </code></pre>
  
  <p>but there is an error:</p>
  
  <pre><code>XGBoostError: [09:48:50] /usr/local/xgboost/src/metric/metric.cc:21: Unknown metric function auc
  
  Stack trace returned 10 entries:
  [bt] (0) /usr/local/lib/python2.7/dist-packages/xgboost-0.6-
  py2.7.egg/xgboost/libxgboost.so(_ZN7xgboost6Metric6CreateERKSs+0x5f7) 
  [0x7f0cadfff417]
  [bt] (1) /usr/local/lib/python2.7/dist-packages/xgboost-0.6-
  py2.7.egg/xgboost/libxgboost.so(_ZN7xgboost11LearnerImpl9ConfigureERKSt6vectorISt4pairISsSsESaIS3_EE+0x717) [0x7f0cae0bfd27]
  [bt] (2) /usr/local/lib/python2.7/dist-packages/xgboost-0.6-py2.7.egg/xgboost/libxgboost.so(XGBoosterUpdateOneIter+0x5e) 
  [0x7f0cadf6816e]
  [bt] (3) /usr/lib/x86_64-linux-gnu/libffi.so.6(ffi_call_unix64+0x4c) 
  [0x7f0d3a3bcadc]
  [bt] (4) /usr/lib/x86_64-linux-gnu/libffi.so.6(ffi_call+0x1fc) [0x7f0d3a3bc40c]
  [bt] (5) /usr/lib/python2.7/lib-dynload/_ctypes.x86_64-linux-gnu.so(_ctypes_callproc+0x48e) [0x7f0d3a5d35fe]
  [bt] (6) /usr/lib/python2.7/lib-dynload/_ctypes.x86_64-linux-gnu.so(+0x15f9e) [0x7f0d3a5d4f9e]
  [bt] (7) /usr/bin/python(PyEval_EvalFrameEx+0x98d) [0x5244dd]
  [bt] (8) /usr/bin/python(PyEval_EvalCodeEx+0x2b1) [0x555551]
  [bt] (9) /usr/bin/python(PyEval_EvalFrameEx+0x7e8) [0x524338]
  </code></pre>