code: def from_string(cls, dataset_id, default_project=None):
          
          output_dataset_id = dataset_id
          output_project_id = default_project
          parts = dataset_id.split(".")
  
          if len(parts) == 1 and not default_project:
              raise ValueError(
                  "When default_project is not set, dataset_id must be a "
                  "fully-qualified dataset ID in standard SQL format. "
                  'e.g. "project.dataset_id", got {}'.format(dataset_id)
              )
          elif len(parts) == 2:
              output_project_id, output_dataset_id = parts
          elif len(parts) > 2:
              raise ValueError(
                  "Too many parts in dataset_id. Expected a fully-qualified "
                  "dataset ID in standard SQL format. e.g. "
                  '"project.dataset_id", got {}'.format(dataset_id)
              )
  
          return cls(output_project_id, output_dataset_id)
docs: Construct a dataset reference from dataset ID string.
  
          Args:
              dataset_id (str):
                  A dataset ID in standard SQL format. If ``default_project``
                  is not specified, this must included both the project ID and
                  the dataset ID, separated by ``.``.
              default_project (str):
                  Optional. The project ID to use when ``dataset_id`` does not
                  include a project ID.
  
          Returns:
              DatasetReference:
                  Dataset reference parsed from ``dataset_id``.
  
          Examples:
              >>> DatasetReference.from_string('my-project-id.some_dataset')
              DatasetReference('my-project-id', 'some_dataset')
  
          Raises:
              ValueError:
                  If ``dataset_id`` is not a fully-qualified dataset ID in
                  standard SQL format.