code: def field_value(self, admin_model, instance, field_name):
          
          _, _, value = lookup_field(field_name, instance, admin_model)
          return value
docs: Returns the value displayed in the column on the web interface for
          a given instance.
  
          :param admin_model:
              Instance of a :class:`admin.ModelAdmin` object that is responsible
              for displaying the change list
          :param instance:
              Object instance that is the row in the admin change list
          :field_name:
              Name of the field/column to fetch