code: def one_or_none(self):
          
          # Sanity check: Has consumption of this query already started?
          # If it has, then this is an exception.
          if self._metadata is not None:
              raise RuntimeError(
                  "Can not call `.one` or `.one_or_none` after "
                  "stream consumption has already started."
              )
  
          # Consume the first result of the stream.
          # If there is no first result, then return None.
          iterator = iter(self)
          try:
              answer = next(iterator)
          except StopIteration:
              return None
  
          # Attempt to consume more. This should no-op; if we get additional
          # rows, then this is an error case.
          try:
              next(iterator)
              raise ValueError("Expected one result; got more.")
          except StopIteration:
              return answer
docs: Return exactly one result, or None if there are no results.
  
          :raises: :exc:`ValueError`: If there are multiple results.
          :raises: :exc:`RuntimeError`: If consumption has already occurred,
              in whole or in part.