Simple range expression parsing.

range_expression.py

import re

def parse_compact_range(expression):
    numbers = set()
    for match_ in re.finditer(r"(?:(\d+)\s*\-\s*(\d+)|(\d+))\s*(?:,|$)", expression):
        start, stop, exact = match_.group(1,2,3)
        if exact is not None:
            numbers.add(int(exact))
        else:
            begin, end = sorted(map(int, (start, stop)))
            numbers.update(range(begin, end + 1))
    return sorted(numbers)


def format_compact_range(*numbers):
    if not numbers:
        return ""
    tokens = []
    nums = sorted(set(numbers))
    start = last_num = nums[0]
    create_token = lambda: tokens.append(
        ("{}".format if start == last_num else "{}-{}".format)(start, last_num)
    )
    for num in nums[1:]:
        if num - 1 != last_num:
            create_token()
            start = num
        last_num = num
    create_token()
    return ",".join(tokens)


if __name__ == "__main__":
    assert [3,6,9] == parse_compact_range("3,6,9")
    assert [1,2,3] == parse_compact_range("1-3")
    assert [1,2,5,6,9] == parse_compact_range("1 - 2, 5 - 6, 9")
    assert [1,2,3] == parse_compact_range("3-1")
    assert [1,2,3,4] == parse_compact_range("1-4, 3, 2, 4-1")
    assert [] == parse_compact_range("")

    assert format_compact_range(3,6,9) == "3,6,9"
    assert format_compact_range(1,2,3) == "1-3"
    assert format_compact_range(1,2,5,6,9) == "1-2,5-6,9"
    assert format_compact_range(1) == "1"
    assert format_compact_range(1,2,3,4) == "1-4"
    assert format_compact_range() == ""