1 3 4 5 7 => 1,3-5,7

hyphen-num.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from itertools import chain, izip_longest, groupby


def hyphenate(s):
    """
    >>> hyphenate("1 2 3")
    '1-3'
    >>> hyphenate("1 2 3 5 7 8")
    '1-3,5,7-8'
    >>> hyphenate("1 3 4 5 7")
    '1,3-5,7'
    """
    arr = [int(c) for c in s.split()]
    is_stepwise = [x - y == -1 for x, y in zip(arr, arr[1:])]
    seq = chain.from_iterable(izip_longest(arr, is_stepwise, fillvalue=False))
    groups = (list(g) for k, g in groupby(seq, bool) if k)
    return ",".join(["{}-{}".format(x[0], x[-1]) if len(x) > 1
                     else "{}".format(x[0]) for x in groups])

if __name__ == '__main__':
    import doctest
    doctest.testmod(verbose=True, report=False)