select.py

from Queue import Queue

def select(source, selector, unselected, sentinel):
    for item in source:
        if selector(item):
            yield item
        else:
            unselected.put(item)
    unselected.put(sentinel)

unselected, sentinel = Queue(), None
selected = select(
    source=(hex(n) for n in xrange(40)),
    selector=lambda h: 'a' not in h,
    unselected=unselected,
    sentinel=sentinel,
)

for h in selected:
    print 'Selected:', h

for h in iter(unselected.get, sentinel):
    print 'Unselected:', h

select_inline.py

from Queue import Queue

unselected = Queue()
selected = (
    item
    for item in (hex(n) for n in xrange(40))
    if ('a' not in item) or unselected.put(item)
)

for h in selected:
    print 'Selected:', h

sentinel = None
unselected.put(sentinel)
for h in iter(unselected.get, sentinel):
    print 'Unselected:', h