zip variable empty after first use
That's how it works in python 3.x. In python2.x, zip returned a list of tuples, but for python3.x, zip behaves like itertools.izip behaved in python2.x. To regain the python2.x behavior, just construct a list from zip's output:
z = list(zip(t,t2))
Note that in python3.x, a lot of the builtin functions now return iterators rather than lists (map, zip, filter)
Because zip returns an iterator in Python 3.x. If you want to re-use it, then make it a list first:
z = list(zip(t, t2))