title: python xgboost continue training on existing model
body: <p>Lets say I build an xgboost model:</p>
  
  <pre><code>bst = xgb.train(param0, dtrain1, num_round, evals=[(dtrain, "training")])
  </code></pre>
  
  <p>Where: </p>
  
  <ul>
  <li>param0 is a set of params to xgb,</li>
  <li>dtrain1 is a DMatrix ready to be trained</li>
  <li>num_round is the number of rounds</li>
  </ul>
  
  <p>Then, I save the model to disk:</p>
  
  <pre><code>bst.save_model("xgbmodel")
  </code></pre>
  
  <p>Later on, I want to reload the model I saved and <strong>continue</strong> training it with dtrain2</p>
  
  <p>Does anyone have an idea how to do it?</p>