list_condense.py

def list_condense(l):
    run_start = None
    in_run = False
    last_item = None
    out = ''

    while len(l):
        item = l.pop(0)
        if not in_run:
            run_start = item
            in_run = True
            out += str(item)
            last_item = item
        else:
            if item == last_item + 1:
                if len(l) == 0:
                    out += '-' + str(item)
                else:
                    last_item = item
            else:
                if last_item - run_start > 0:
                    out += '-' + str(last_item)
                out += ','
                l.insert(0,item)
                in_run = False

    return out


if __name__ == "__main__":

    examples = [[1,2,3,5,7,8,10],
                [1,3,5,7,9],
                [1,3,5,6,8,9],
                [1,2,5,6,7],
                range(20)]

    for example in examples:
        print example
        print list_condense(example)
        print

    # OUTPUT
    # [1, 2, 3, 5, 7, 8, 10]
    # 1-3,5,7-8,10

    # [1, 3, 5, 7, 9]
    # 1,3,5,7,9

    # [1, 3, 5, 6, 8, 9]
    # 1,3,5-6,8-9

    # [1, 2, 5, 6, 7]
    # 1-2,5-7

    # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
    # 0-19

Good.