Gets both numbers adjacent to input number.

get_neighbours.py

import bisect


def get_neighbours(num, interval):
    interval = set(interval)
    interval.discard(num)
    interval = list(interval)
    interval.sort()
    
    if num < interval[0] or interval[-1] < num:
        return interval[-1], interval[0]

    else:
        i = bisect.bisect_right(interval, num)
        neighbours = interval[i-1:i+1]
        return neighbours


### test input
l = [1, 3, 4, 8, 20, 33, 36, 68]
    
>>> get_neighbours(8, l)  # element in l
4, 20

>>> get_neighbours(55, l) # element not in l
36, 68