range_extraction.py

"""
A format for expressing an ordered list of integers is to use a comma separated list of either

individual integers or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, '-'. 
The range includes all integers in the interval including both endpoints. 
It is not considered a range unless it spans at least 3 numbers. For example "12,13,15-17"
Complete the solution so that it takes a list of integers in increasing order and returns a correctly formatted string in the range format.

Example:
    solution([-10, -9, -8, -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20])
    # returns "-10--8,-6,-3-1,3-5,7-11,14,15,17-20"

Courtesy of rosettacode.org
"""


def solution(args):
    output = []
    range_seq = []

    head, *tail = args
    range_seq.append(head)

    for n in tail+[None]:
        if n != range_seq[-1] + 1:
            if len(range_seq) > 2:
                output.append(f'{range_seq[0]}-{range_seq[-1]}')
            else:
                output += [str(r) for r in range_seq]
            range_seq = [n]
        else:
            range_seq.append(n)
    return ','.join(output)