title: Which decision_function_shape for sklearn.svm.SVC when using OneVsRestClassifier?
body: <p>I am doing multi-label classification where I am trying to predict correct tags to questions:</p>
  
  <p>(X = questions, y = list of tags for each question from X).</p>
  
  <p>I am wondering, which <code>decision_function_shape</code> for <a href="http://scikit-learn.org/stable/modules/generated/sklearn.svm.SVC.html" rel="nofollow noreferrer"><code>sklearn.svm.SVC</code></a> should be be used with <a href="http://scikit-learn.org/stable/modules/generated/sklearn.multiclass.OneVsRestClassifier.html" rel="nofollow noreferrer"><code>OneVsRestClassifier</code></a>?</p>
  
  <p>From docs we can read that <code>decision_function_shape</code> can have two values <code>'ovo'</code> and <code>'ovr'</code>:</p>
  
  <blockquote>
    <p><strong>decision_function_shape</strong> : ‘ovo’, ‘ovr’ or None, default=None</p>
    
    <p>Whether to return a one-vs-rest (‘ovr’) decision function of shape (n_samples, n_classes) as all other classifiers, or the original
    one-vs-one (‘ovo’) decision function of libsvm which has shape
    (n_samples, n_classes * (n_classes - 1) / 2). The default of None will
    currently behave as ‘ovo’ for backward compatibility and raise a
    deprecation warning, but will change ‘ovr’ in 0.19.</p>
  </blockquote>
  
  <p>But I still don't understand what is the difference between:</p>
  
  <pre><code># First decision_function_shape set to 'ovo'
  estim = OneVsRestClassifier(SVC(kernel='linear', decision_function_shape ='ovo'))
  
  # Second decision_function_shape set to 'ovr'
  estim = OneVsRestClassifier(SVC(kernel='linear', decision_function_shape ='ovr'))
  </code></pre>
  
  <p>Which <code>decision_function_shape</code> should be used for <a href="http://scikit-learn.org/stable/modules/multiclass.html#multiclass-and-multilabel-algorithms" rel="nofollow noreferrer">multi-label classification</a> problem?</p>
  
  <p><strong>EDIT:</strong> <a href="https://stackoverflow.com/questions/39604468/what-is-the-difference-between-onevsrestclassifier-with-svc-and-svc-with-decisio?rq=1">Question</a> asking a similar thing with no answer.</p>