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

nth.py

from itertools import izip_longest, izip, tee
import random

# Similar to itertools.islice
class Nth(object):
    def __init__(self, n):
        self.n = n
        self.i = 0
        self.nout = 0

    def itervalues(self, x):
        self.i = 0
        self.nout = 0
        for xi in x:
            self.i += 1
            if self.i == self.n:
                self.i = 0
                self.nout += 1
                yield self.nout, xi

def add(x,y):
    for xi,yi in izip(x,y):
        yield xi + yi

def sub(x,y):
    for xi,yi in izip(x,y):
        yield xi - yi

class NthSumDiff(object):
    def __init__(self, n):
        self.nthsum = Nth(n)
        self.nthdiff = Nth(n)

    def itervalues(self, x, y):
        xadd, xsub = tee(x)
        yadd, ysub = tee(y)
        gen_sum = self.nthsum.itervalues(add(xadd, yadd))
        gen_diff = self.nthdiff.itervalues(sub(xsub, ysub))
        # Have to use izip_longest here, but why?
        #for (i,nthsum), (j,nthdiff) in izip_longest(gen_sum, gen_diff):
        for (i,nthsum), (j,nthdiff) in izip(gen_sum, gen_diff):
            assert i==j, "sum row %d != diff row %d" % (i,j)
            yield nthsum, nthdiff

nskip = 12
ns = Nth(nskip)
nd = Nth(nskip)
nsd = NthSumDiff(nskip)
nfiles = 10
for i in range(nfiles):
    # Generate some data.
    # If the block length is a multiple of nskip there's no problem.
    #n = random.randint(5000, 10000) * nskip
    n = random.randint(50000, 100000)
    print 'file %d n=%d' % (i, n)
    x = range(n)
    y = range(100,n+100)
    # Independent processing is no problem but requires two loops.
    for i, nthsum in ns.itervalues(add(x,y)):
        pass
    for j, nthdiff in nd.itervalues(sub(x,y)):
        pass
    assert i==j
    # Trying to do both with one loops causes problems.
    for nthsum, nthdiff in nsd.itervalues(x,y):
        # If izip_longest is necessary, why don't I ever get a fillvalue?
        assert nthsum is not None
        assert nthdiff is not None
    # After each block of data the two iterators should have the same state.
    assert nsd.nthsum.nout == nsd.nthdiff.nout, \
           "sum nout %d != diff nout %d" % (nsd.nthsum.nout, nsd.nthdiff.nout)