Question

I was wondering that whether there’s a way to refactor the following codes

 first_run = True
 for i in gen:
        if first_run:
            last_head = i[1]
            last_tail = i[2]
            last_chrom = i[0]
            first_run = False
        else:
            func(i[1], last_head)
            func(i[1], last_tail)
            last_head = i[1]
            last_tail = i[2]
            last_chrom = i[0]

Answer 1

The essential point of your loop seems to be performing some operation on pairs of consecutive elements of the iterable. So I would look to the function pairwise whose definition is given in the itertools module documentation:

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)

Note that this is not an actual itertools function, you will have to copy and paste the implementation into your code. Anyway, with this function, your loop can be implemented like so:

for a, b in pairwise(gen):
    func(b[1], a[1])
    func(b[1], a[2])