def format_sklearn(self):
        """
        Returns dataset in (X, y) format for use in scikit-learn.
        Unlabeled entries are ignored.

        Returns
        -------
        X : numpy array, shape = (n_samples, n_features)
            Sample feature set.

        y : numpy array, shape = (n_samples)
            Sample labels.
        """
        X, y = zip(*self.get_labeled_entries())
        return np.array(X), np.array(y)