How to chunk a list in Python 3?

Question

I found the following code that is compatible with python2

from itertools import izip_longest
def grouper(n, iterable, padvalue=None):
  \"grouper(3, \'abcde         


        

Answer 1

In Python 3's itertools there is a function called zip_longest. It should do the same as izip_longest from Python 2.

Why the change in name? You might also notice that itertools.izip is now gone in Python 3 - that's because in Python 3, the zip built-in function now returns an iterator, whereas in Python 2 it returns a list. Since there's no need for the izip function, it also makes sense to rename the _longest variant for consistency.

Answer 2

According to the doc:

>>> s = [1,2,3,4,5,6,7,8,9]
>>> n = 3
>>> list(zip(*[iter(s)]*n))
[(1, 2, 3), (4, 5, 6), (7, 8, 9)]

Answer 3

After all that discussion above, here's a python3 solution that I believe gives safer, more predicable results.

def chunker(iter, size):
    chunks = [];
    if size < 1:
        raise ValueError('Chunk size must be greater than 0.')
    for i in range(0, len(iter), size):
        chunks.append(iter[i:(i+size)])
    return chunks

example = [1,2,3,4,5,6,7,8,9]
print(' 1: ' + str(chunker(example, 1)))
print(' 3: ' + str(chunker(example, 3)))
print(' 4: ' + str(chunker(example, 4)))
print(' 8: ' + str(chunker(example, 8)))
print(' 9: ' + str(chunker(example, 9)))
print('10: ' + str(chunker(example, 10)))

The results are:

$ python3 iter_chunk.py 
 1: [[1], [2], [3], [4], [5], [6], [7], [8], [9]]
 3: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
 4: [[1, 2, 3, 4], [5, 6, 7, 8], [9]]
 8: [[1, 2, 3, 4, 5, 6, 7, 8], [9]]
 9: [[1, 2, 3, 4, 5, 6, 7, 8, 9]]
10: [[1, 2, 3, 4, 5, 6, 7, 8, 9]]