Answer for [Can I yield from an instance method](http://stackoverflow.com/q/33818422/47078)

nth.py

from itertools import izip, islice
import random

def sumdiff(data):
    return ((x + y, x - y) for x, y in data)

def combined_file_data(files):
    for i,n in files:
        # Generate some data.
        x = range(n)
        y = range(100,n+100)
        for data in izip(x,y):
            yield data

def filelist(nfiles):
    for i in range(nfiles):
        # Generate some data.
        n = random.randint(50000, 100000)
        print 'file %d n=%d' % (i, n)
        yield i, n

def Nth(iterable, step):
    return islice(iterable, step-1, None, step)

nskip = 12
nfiles = 10
filedata = combined_file_data(filelist(nfiles))
nth_data = Nth(filedata, nskip)
for nthsum, nthdiff in sumdiff(nth_data):
    assert nthsum is not None
    assert nthdiff is not None