Cycling through iterables diagonally

[Peter Otten]

Pablo Lucena wrote:

> Say I have a group of 4 lists as follows:
> 
> l1 = ['a1', 'a2', 'a3', 'a4']
> l2 = ['b1', 'b2', 'b3', 'b4']
> l3 = ['c1', 'c2', 'c3', 'c4']
> l4 = ['d1', 'd2', 'd3', 'd4']
> 
> I would like to cycle through these lists "diagonally" in groups of
> len(list) (in this example, each list has 4 items).

> Prior to this I was mucking around with index counting while looping, and
> popping lists out of a deque, popping an item out of the list, and
> appending the list back into the deque during each iteration.
> 
> Is there a better/cleaner way to do this? I was hoping for some cool
> itertools logic =)

I have a weak spot for the itertools myself, but I think in terms of clarity 
it is hard to beat the conventional

def diagonals(a):
    N = len(a)
    for i in range(N):
        for k in range(N):
            yield a[k][(k+i)%N]

print(list(diagonals([l1, l2, l3, l4])))

Of course that's as uncool as it can get ;)