title: Difference between evaluation metrics and evaluation function in lightgbm
body: <p>Refer to <a href="https://lightgbm.readthedocs.io/en/latest/Python-API.html#lightgbm.cv" rel="nofollow noreferrer">lightgbm.cv</a>, there are two parameters confusing me: <code>metrics</code> and <code>feval</code>. Based on my little knowledge on gbdt, evaluation metrics and evaluation function are both to compute the loss, such as auc, from a vector of predictions and a vector of true labels.  </p>
  
  <p>However, <code>metrics</code> and <code>function</code> sound like they are handling different tasks .</p>
  
  <ol>
  <li>Are they doing the same thing?</li>
  <li>To be more specific, if I execute <code>lightgbm.cv(params,metrics='auc', feval='ks')</code>, will <code>feval='ks'</code> override <code>metrics='auc'</code>?</li>
  </ol>