def get_span_string(run0, run1, runs=None):
    """Returns string of run0-run1, (or run0 if run0 == run1)
    """
    if runs is not None:
        string = ''
        for run in runs:
            string += f'{run},'
        return string

    elif run0 == run1:
        return f'{run0}'
    else:
        return f'{run0}-{run1}'