def _sample_field(self, sample):
        """Returns string representation of sample-format values.

        Raises:
            KeyError: if requested sample is not defined.
        """
        tag_values = self.sample_tag_values[sample].values()
        if tag_values:
            return ":".join(tag_values)
        else:
            return "."
Returns string representation of sample-format values.

        Raises:
            KeyError: if requested sample is not defined.