>>> line = '1234567890'
>>> n = 2
>>> [line[i:i+n] for i in range(0, len(line), n)]
['12', '34', '56', '78', '90']

One solution would be to use this function:

def chunkstring(string, length):
    return (string[0+i:length+i] for i in range(0, len(string), length))

This function returns a generator, using a generator comprehension. The generator returns the string sliced, from 0 + a multiple of the length of the chunks, to the length of the chunks + a multiple of the length of the chunks.

You can iterate over the generator like a list, tuple or string - for i in chunkstring(s,n): , or convert it into a list (for instance) with list(generator). Generators are more memory efficient than lists because they generator their elements as they are needed, not all at once, however they lack certain features like indexing.

This generator also contains any smaller chunk at the end:

>>> list(chunkstring("abcdefghijklmnopqrstuvwxyz", 5))
['abcde', 'fghij', 'klmno', 'pqrst', 'uvwxy', 'z']

Example usage:

text = """This is the first line.
           This is the second line.
           The line below is true.
           The line above is false.
           A short line.
           A very very very very very very very very very long line.
           A self-referential line.
           The last line.
        """

lines = (i.strip() for i in text.splitlines())

for line in lines:
    for chunk in chunkstring(line, 16):
        print(chunk)

>>> s = '25c319f75e3fbed5a9f0497750ea12992b30d565'
>>> n = [8, 4, 4, 4, 4, 12]
>>> print '-'.join([s[sum(n[:i]):sum(n[:i+1])] for i in range(len(n))])

Output

25c319f7-5e3f-bed5-a9f0-4977-50ea12992b30

You can use x[start_index:end_index] which yields a substring from x (last index excluded). So just specify 2 positions and use them e.g.:

a = len(x) // 3
b = 2* len(x) // 3
s0 = x[:a]
s1 = x[a:b]
s2 = x[b:]

If you'd like to access a string 3 characters at a time, you're going to need to use slicing.

You can get a list of the 3-character long pieces of the string using a list comprehension like this:

>>> x = 'this is a string'
>>> step = 3
>>> [x[i:i+step] for i in range(0, len(x), step)]
['thi', 's i', 's a', ' st', 'rin', 'g']
>>> step = 5
>>> [x[i:i+step] for i in range(0, len(x), step)]
['this ', 'is a ', 'strin', 'g']

The important bit is:

[x[i:i+step] for i in range(0, len(x), step)]

range(0, len(x), step) gets us the indices of the start of each step-character slice. for i in will iterate over these indices. x[i:i+step] gets the slice of x that starts at the index i and is step characters long.

If you know that you will get exactly four pieces every time, then you can do:

a, b, c, d = [x[i:i+step] for i in range(0, len(x), step)]

This will happen if 3 * step < len(x) <= 4 * step.

If you don't have exactly four pieces, then Python will give you a ValueError trying to unpack this list. Because of this, I would consider this technique very brittle, and would not use it.

You can simply do

x_pieces = [x[i:i+step] for i in range(0, len(x), step)]

Now, where you used to access a, you can access x_pieces[0]. For b, you can use x_pieces[1] and so on. This allows you much more flexibility.

a = [1, 2, 3, 4, 5, 6, 7, 8, 9]
CHUNK = 4
[a[i*CHUNK:(i+1)*CHUNK] for i in xrange((len(a) + CHUNK - 1) / CHUNK )]