Not sure how crucial this exactly is, but the
itertools.izip() can be abused to create a tuple that
can be mutated once. I couldn't find a way to crash
anything, however, but I wouldn't leave such a
possibility open in the Python interpreter nevertheless.

. 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)