title: A column-vector y was passed when a 1d array was expected
body: <p>I need to fit <code>RandomForestRegressor</code> from <code>sklearn.ensemble</code>.</p>
  <pre><code>forest = ensemble.RandomForestRegressor(**RF_tuned_parameters)
  model = forest.fit(train_fold, train_y)
  yhat = model.predict(test_fold)
  </code></pre>
  <p>This code always worked until I made some preprocessing of data (<code>train_y</code>).
  The error message says:</p>
  <blockquote>
  <p>DataConversionWarning: A column-vector y was passed when a 1d array was expected. Please change the shape of y to (n_samples,), for example using ravel().</p>
  <p>model = forest.fit(train_fold, train_y)</p>
  </blockquote>
  <p>Previously <code>train_y</code> was a Series, now it's numpy array (it is a column-vector). If I apply <code>train_y.ravel()</code>, then it becomes a row vector and no error message appears, through the prediction step takes very long time (actually it never finishes...).</p>
  <p>In the docs of <code>RandomForestRegressor</code> I found that <code>train_y</code> should be defined as <code>y : array-like, shape = [n_samples] or [n_samples, n_outputs]</code>
  Any idea how to solve this issue?</p>