Python-сообщество

    r = []
    for t in t_old:
        if t in t_new: r.append(t)
    for t in r:
        del t_old[t]
        del t_new[t]

    t_old = []
    for t in t_old1:
        if t not in t_new1: t_old.append(t)
    t_new = []
    for t in t_new1:
        if t not in t_old1: t_new.append(t)

>>> l1 = [1,2,3]
>>> l2 = [3,4,5]
>>> s = set(l1) & set(l2)
>>> s
set([3])
>>> r1 = [i for i in l1 if i not in s]
>>> r1 
[1, 2]
>>> r2 = [i for i in l2 if i not in s]
>>> r2
[4, 5]

list(set(l1) - set(l2))
list(set(l2) - set(l1))

r1 = filter(lambda x: x not in l2, l1)
r2 = filter(lambda x: x not in l1, l2)