I'm sorry, is this a joke? "straightforward"? Why would you use that code when you could implement the same thing more simply like this?

    def line_number_mogrifier(f):
        num = 0
        for line in f:
            yield (num, line)
            num += 1

The functional proponents say that eventually everything will be functional, I guess I'll wait and see how that goes over. People think procedurally. Functional constructs may have some technical advantages, but if adopting them shrinks the pool of effective developers, it won't catch on.