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How to Iterate Over an Object by Chunks

The for loop allows you to go through the elements of an iterable object one at a time:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for n in numbers:
    print(n)

Using the batched() function, you can loop through two or more elements:

# even receives a tuple of two elements.
for pair in batched(numbers, 2):
    print(pair)

This prints:

(1, 2) (3, 4) (5, 6) (7, 8) (9, 10)

Another example: If the number of elements of the iterated object is not multiple of the second argument, then in the last iteration a tuple with fewer elements is received:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
for pair in batched(numbers, 2):
    print(pair)

Output:

(1, 2) (3, 4) (5, 6) (7, 8) (9, 10) (11,)

To iterate by chunks of 3 items:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
for triplet in batched(numbers, 3):
     print(triplet)

Output

(1, 2, 3) (4, 5, 6) (7, 8, 9) (10, 11, 12)

Here's the batched() function implementation (borrowed from Python recipes):

from itertools import islice

def batched(iterable, n):
    "Batch data into tuples of length n. The last batch may be shorter."
    # batched('ABCDEFG', 3) --> ABC DEF G
    if n < 1:
        raise ValueError('n must be at least one')
    it = iter(iterable)
    while batch := tuple(islice(it, n)):
        yield batch

Starting from Python 3.12, batched() is shipped within the itertools standard module:

from itertools import batched

iterators iterables loops lists tuples dictionaries

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