Return the polarity score as a float within the range [-1.0, 1.0]

def polarity(self):
    """Return the polarity score as a float within the range [-1.0, 1.0]
    """
    scores = [w.polarity for w in self.words if w.polarity != 0]
    if not scores:
      return 0.0
    return sum(scores) / float(len(scores))