title: Is it possible to toggle a certain step in sklearn pipeline?
body: <p>I wonder if we can set up an "optional" step in <code>sklearn.pipeline</code>. For example, for a classification problem, I may want to try an <code>ExtraTreesClassifier</code> with AND without a <code>PCA</code> transformation ahead of it. In practice, it might be a pipeline with an extra parameter specifying the toggle of the <code>PCA</code> step, so that I can optimize on it via <code>GridSearch</code> and etc. I don't see such an implementation in sklearn source, but is there any work-around?</p>
  
  <p>Furthermore, since the possible parameter values of a following step in pipeline might depend on the parameters in a previous step (e.g., valid values of <code>ExtraTreesClassifier.max_features</code> depend on <code>PCA.n_components</code>), is it possible to specify such a conditional dependency in <code>sklearn.pipeline</code> and <code>sklearn.grid_search</code>?</p>
  
  <p>Thank you!  </p>