code: def append(self, feature, label=None):
          
          self.data.append((feature, label))
          self.modified = True
          return len(self.data) - 1
docs: Add a (feature, label) entry into the dataset.
          A None label indicates an unlabeled entry.
  
          Parameters
          ----------
          feature : {array-like}, shape = (n_features)
              Feature of the sample to append to dataset.
  
          label : {int, None}
              Label of the sample to append to dataset. None if unlabeled.
  
          Returns
          -------
          entry_id : {int}
              entry_id for the appened sample.