# yield_combos.py

# yield_combos.py

def combos(iterable, r):
    iter = tuple(iterable)
    n = len(iter)
    if r > n:
        return
    indices = range(r)
    yield tuple(iter[i] for i in indices)
    while True:
        for i in reversed(range(r)):
            if indices[i] != i + n - r:
                break
        else:
            return
        indices[i] += 1
        for j in range(i+1, r):
            indices[j] = indices[j-1] + 1
        yield tuple(iter[i] for i in indices)

z = combos('ABCD', 2) # --> AB AC AD BC BD CD
while 1:
    try:
        print next(z)
    except:
        break
for z in combos(range(4), 3): # --> 012 013 023 123
    print z