title: Labels (y) in Scikit-learn Pipeline and Compose Classes
body: <p>My current understanding is that, we cant directly transform/retrieve <code>y-labels</code> passed as <code>(X,y)</code> while using a Pipeline.</p>
  <p>The <code>fit_transform</code> at the end returns transformations only on the <code>X</code> passed and <code>y</code> is only utilized in situations involving <code>fit()</code>, <code>fit_predict()</code> and such.</p>
  <p>Is my understanding correct?</p>
  <p>Also is there a way to transform and retrieve <code>y</code> (including when dropping instances using a Custom Transformer) without having to break out of a fully enclosed model training pipeline?</p>