groupby

from operator import itemgetter
from itertools import *
#
pts = IN[0]
ptX = IN[1]
out_grp = []
list1 = zip(pts,ptX)
list1.sort(key=itemgetter(1))
for i, g in groupby(list1, lambda x: x[1]):
    out_grp.append(list(g))
OUT = out_grp