from itertools import groupby
from operator import itemgetter


thelist = [1, 2, 3, 2, 4, 5, 9, 7, 8, 9, 10, 4, 5, 6]
newlist = []

for k, g in groupby(enumerate(thelist), lambda ix: ix[0] - ix[1]):
    newlist.append(list(map(itemgetter(1), g)))

print(max(newlist, key=lambda x: len(x)))