comparing two sorted DBAPI rowsets in python

iter_comm.py

SOURCE_LEFT, SOURCE_RIGHT, SOURCE_BOTH = 'left', 'right', 'both'

def comm(itera, iterb, cmpfunc=None):
    """ merge two iterators and indicate which items are common (similar
    to the unix 'comm' command).
    
    Return an iterator containing deduped union of two sorted iterables and 
    a string indicating whether merged values are unique to the 'left' iterator, 
    'right' iterator or if they are common to both iterators.

    
    itera -- the 'left' iterable
    iterb -- the 'right' iterable
    cmpfunc -- Optional comparator. If none supplied, use natural order.
    """

    cmp = cmpfunc or (lambda a,b: (a > b) - (a < b))
    done = object()
    anext = next(itera, done)
    bnext = next(iterb, done)

    while (anext is not done) or (bnext is not done):
        if bnext is done or (anext is not done and cmp(anext, bnext) < 0):
            # left column entry is unique
            yield anext, SOURCE_LEFT
            anext = next(itera, done)
        elif  anext is done or (bnext is not done and cmp(anext, bnext) > 0):
            # right column entry is unique
            yield bnext, SOURCE_RIGHT
            bnext = next(iterb, done)
        else:
            if bnext is not done and anext is not done and cmp(anext, bnext) == 0:
                # value is common to both iterators
                yield bnext, SOURCE_BOTH 
                anext = next(itera, done)
                bnext = next(iterb, done)