from itertools import imap, izip

def mutatingtuple(tuple1, f, tuple2):
    # this builds a tuple t which is a copy of tuple1,
    # then calls f(t), then mutates t to be equal to tuple2
    # (needs len(tuple1) == len(tuple2)).
    def g(value, first=[1]):
        if first:
            del first[:]
            f(z.next())
        return value
    items = list(tuple2)
    items[1:1] = list(tuple1)
    gen = imap(g, items)
    z = izip(*[gen]*len(tuple1))
    z.next()

def f(t):
    global T
    T = t
    print T

mutatingtuple((1,2,3), f, (4,5,6)) # print T -> (1, 2, 3)
print T                            # print T -> (4, 5, 6)