docs: def add_feature(self, label, value=None):
          """
          label: A VW label (not containing characters from escape_dict.keys(),
              unless 'escape' mode is on)
          value: float giving the weight or magnitude of this feature
          """
          if self.escape:
              label = escape_vw_string(label)
          elif self.validate:
              validate_vw_string(label)
          feature = (label, value)
          self.features.append(feature)