Format consecutive numbers as ranges

format_consecutive.py

from itertools import count, groupby
from typing import Iterable, Tuple


def group_consecutive(nums: Iterable[int]) -> Iterable[Tuple[int, ...]]:
    counter = count()
    for _, g_it in groupby(nums, lambda n: n - next(counter)):
        yield tuple(g_it)


def format_consecutive_group(group: Tuple[int, ...], sep: str = "-") -> str:
    if len(group) == 1:
        return f"{group[0]}"
    return f"{group[0]}{sep}{group[-1]}"


def format_consecutive(
    nums: Iterable[int], sep: str = ",", range_sep: str = "-"
) -> str:
    """
    >>> format_consecutive([1, 2, 3, 4, 8, 9, 10, 100, 200])
    "1-4,8-10,100,200"
    """
    groups = group_consecutive(sorted(nums))
    return sep.join(format_consecutive_group(g, sep=range_sep) for g in groups)