docs: def perform_transformation(instance, field_names_to_transform=None):
      '''
      Transforms a model based on the fields specified in the
      ``transformed_fields`` attribute. This should map source image
      field names to dictionaries mapping destination field name to
      their transformations. For instance::
  
          {
              'image': {
                  'thumbnail': scale(width=10),
              }
          }
  
      If ``field_names_to_transform`` is None, then it will transform
      all fields. Otherwise it will only transform
      from those fields specified in ``field_names_to_transform``.
  
      :param instance: model instance to perform transformations on
      :type instance: instance of :py:class:`django.db.models.Model`
  
      :param field_names_to_transform: field names on model to perform transformations on
      :type field_names_to_transform: iterable of strings or None
      '''
      for source_field_name, destination_dict in instance.transformed_fields.items():
          if field_names_to_transform is None or source_field_name in field_names_to_transform:
              for destination_field_name, transformation in destination_dict.items():
                  arguments = [instance, source_field_name, destination_field_name, transformation]
                  get_caller()(transform_field, *arguments)