code: def variables(self, include_units=False):
          
          if include_units:
              return self.data[['variable', 'unit']].drop_duplicates()\
                  .reset_index(drop=True).sort_values('variable')
          else:
              return pd.Series(self.data.variable.unique(), name='variable')
docs: Get a list of variables
  
          Parameters
          ----------
          include_units: boolean, default False
              include the units