>>> n = 3
>>> x = range(n ** 2),
>>> xn = list(zip(*[iter(x)] * n))

In PEP 618, the author gives this example of how zip can be used to chunk data into equal sized groups.

How does it work?

I think that it relies on an implementation detail of zip such that if it takes the first element of each of the elements of the list [iter(x)] * n that equates to the first n elements because of the changing state of iter(x) as each of the elements are taken.

This is because the following code replicates the above behavior:

n = 3
x = range(n ** 2)
xn = [iter(x)] * n

res = []

while True:    
                col = []    
                for element in xn:    

However, I would like to make sure that this is indeed the case and that this is a reliable behavior that can be used to chunk elements of an iterable.


It's not really specific to zip, but you basically have that right. In effect, it's zipping 3 references to the same iterator, causing it to round-robin between them. During each iteration, one more element is consumed from the iterator.

Effectively, it's the same as doing this:

>>> n = 3
>>> x = range(n ** 2)
>>> a = b = c = iter(x)
>>> list(zip(a, b, c))
[(0, 1, 2), (3, 4, 5), (6, 7, 8)]

Note that it only produces equal sized groups and may drop elements (that part is a characteristic of zip, because it's limited by the smallest iterable, though you could use itertools.zip_longest if you want):

>>> n = 4
>>> x = range(n ** 2)
>>> a = b = c = iter(x)
>>> list(zip(a, b, c))
[(0, 1, 2), (3, 4, 5), (6, 7, 8), (9, 10, 11), (12, 13, 14)]


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