code: def clean(self, data):
          
          try:
              value = data[self.column_name]
          except KeyError:
              raise KeyError("Column '%s' not found in dataset. Available "
                             "columns are: %s" % (self.column_name, list(data)))
  
          # If ValueError is raised here, import_obj() will handle it
          value = self.widget.clean(value, row=data)
  
          if value in self.empty_values and self.default != NOT_PROVIDED:
              if callable(self.default):
                  return self.default()
              return self.default
  
          return value
docs: Translates the value stored in the imported datasource to an
          appropriate Python object and returns it.