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1. Get our chunks.

2. Round robin through them.

3. Join the result

   def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in range(0, len(l), n):
    yield l[i:i+n]
   def encrypt(s):
    n = int(math.sqrt(len(s))
    chunked = chunks(s, n)
    return ''.join(roundrobin(*chunked))
   

1. Get our chunks.

2. Round robin through them.

3. Join the result

   def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in range(0, len(l), n):
    yield l[i:i+n]
   def encrypt(s):
    n = int(math.sqrt(len(s))
    chunked = chunks(s, n)
    return ''.join(roundrobin(*chunked))
   

1. Get our chunks.

2. Round robin through them.

3. Join the result

   def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in xrange(0, len(l), n):
    yield l[i:i+n]
   def encrypt(s):
    n = int(math.sqrt(len(s))
    chunked = chunks(s, n)
    return ''.join(roundrobin(*chunked))
   

1. Get our chunks.

2. Round robin through them.

3. Join the result

   def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in range(0, len(l), n):
    yield l[i:i+n]
   def encrypt(s):
    n = int(math.sqrt(len(s))
    chunked = chunks(s, n)
    return ''.join(roundrobin(*chunked))
   

That's precisely what zip_longest is for!

1. Get our chunks.

2. Zip through them.

3. Join the result

   def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in xrange(0, len(l), n):
    yield l[i:i+n]
   def encrypt(s):
    n = int(math.sqrt(len(s))
    chunked = chunks(s, n)
    zipped = itertools.zip_longest(*chunked, fillvalue='')
     return ''.join(''.join(v) for v in zipped)
   

That is, we're round robining through each row. There's a recipe for that:

def roundrobin(*iterables):
 "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
 # Recipe credited to George Sakkis
 pending = len(iterables)
 nexts = cycle(iter(it).__next__ for it in iterables)
 while pending:
 try:
 for next in nexts:
 yield next()
 except StopIteration:
 pending -= 1
 nexts = cycle(islice(nexts, pending))

1. Get our chunks.

2. Round robin through them.

3. Join the result

   def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in xrange(0, len(l), n):
    yield l[i:i+n]
   def encrypt(s):
    n = int(math.sqrt(len(s))
    chunked = chunks(s, n)
    return ''.join(roundrobin(*chunked))