code: def field_dict(self, model):
          
          if self.field_names is None:
              return dict([(f.name, f) for f in model._meta.fields
                           if isinstance(f, models.DateField)])
          else:
              return dict([(f.name, f)
                           for f in model._meta.fields
                           if isinstance(f, models.DateField) and
                              (f.name in self.field_names)])
docs: Helper function that returns a dictionary of all DateFields or
          DateTimeFields in the given model. If self.field_names is set,
          it takes that into account when building the dictionary.