title: difference between custom_loss and custom_metric in catboost CatBoostClassifier
body: <p>I am trying to learn catboost, and I see two confusing terms with CatBoostClassifier:</p>
  <p><code>custom_loss</code> and <code>custom_metric</code>.</p>
  <p>I have browsed here which says: <a href="https://catboost.ai/docs/concepts/python-reference_parameters-list.html#python-reference_parameters-list" rel="nofollow noreferrer">https://catboost.ai/docs/concepts/python-reference_parameters-list.html#python-reference_parameters-list</a></p>
  <p>custom_metric:
  <code>Metric values to output during training. These functions are not optimized and are displayed for informational purposes only. Some metrics support optional parameters (see the Objectives and metrics section for details on each metric).</code></p>
  <p>but then what is <code>custom_loss</code>?</p>
  <p>I see <code>custom_loss</code> defined in the R documentation: <a href="https://catboost.ai/docs/features/loss-functions-desc.html" rel="nofollow noreferrer">https://catboost.ai/docs/features/loss-functions-desc.html</a>   - but not in the python one.</p>
  <p>yet. on the python <a href="https://render.githubusercontent.com/view/ipynb?color_mode=auto&amp;commit=2a1fd8a46d94f5f93672919edb66f568a06c9821&amp;enc_url=68747470733a2f2f7261772e67697468756275736572636f6e74656e742e636f6d2f636174626f6f73742f7475746f7269616c732f326131666438613436643934663566393336373239313965646236366635363861303663393832312f707974686f6e5f7475746f7269616c2e6970796e62&amp;nwo=catboost%2Ftutorials&amp;path=python_tutorial.ipynb&amp;repository_id=138584767&amp;repository_type=Repository#2.1-Model-Training" rel="nofollow noreferrer">tutorial</a>, they have defined a <code>custom_loss</code> like so:</p>
  <pre><code>model = CatBoostClassifier(
      custom_loss=['Accuracy'],
      random_seed=42,
      logging_level='Silent'
  )
  </code></pre>
  <p>Am I missing something here? Infact, <code>custom_loss</code> does not seem to be defined as a property anywhere in the python docs: <a href="https://catboost.ai/docs/concepts/python-reference_parameters-list.html#python-reference_parameters-list" rel="nofollow noreferrer">https://catboost.ai/docs/concepts/python-reference_parameters-list.html#python-reference_parameters-list</a></p>