Python for making 1, 2 … 5, 6, 7 … 10, 11 pagination correctly.

paginator.py

def pages(n, max, window=1, ellipsis=None):
    s = set(range(window + 2))
    s.update(range(n-window, n+window+1))
    s.update(range(max-window, max+1))
    seq = [i for i in sorted(s) if i > 0 and i <= max]
    res = [1]
    for i, nxt in zip(seq, seq[1:]):
        diff = nxt - i
        if diff == 2:
            res.append(i+1)
        elif diff > 2:
            res.append(ellipsis)

        res.append(nxt)

    return res