Loads bond length data from json file

def _load_bond_length_data():
    """Loads bond length data from json file"""
    with open(os.path.join(os.path.dirname(__file__),
                           "bond_lengths.json")) as f:
        data = collections.defaultdict(dict)
        for row in json.load(f):
            els = sorted(row['elements'])
            data[tuple(els)][row['bond_order']] = row['length']
        return data