Split a list into chunks in Python

Splitting a list into chunks means cutting it into smaller lists of the same size. The last chunk is shorter when the items do not divide evenly.

The program

def chunk(items, size):
    if size < 1:
        raise ValueError("size must be at least 1")
    chunks = []
    for start in range(0, len(items), size):
        chunks.append(items[start:start + size])
    return chunks

players = ["Ada", "Linus", "Grace", "Alan", "Hedy", "Margaret", "Guido"]

print(chunk(players, 3))
print(chunk([1, 2, 3, 4], 2))
print(chunk([], 3))

[['Ada', 'Linus', 'Grace'], ['Alan', 'Hedy', 'Margaret'], ['Guido']]
[[1, 2], [3, 4]]
[]

How it works

• range(0, len(items), size) counts up in steps of size. For 7 players and a size of 3, it gives the start positions 0, 3 and 6.
• items[start:start + size] cuts out one chunk. A slice that goes past the end of the list stops at the end without an error, so the last chunk is ['Guido'].
• An empty list gives no start positions, so the result is an empty list.
• A size of 0 makes range() raise ValueError: range() arg 3 must not be zero, and a negative size returns an empty list with no error. The function checks the size first and gives a clear message.

Another way: itertools.batched()

Python 3.12 added batched() to the itertools module. It gives each chunk as a tuple:

from itertools import batched

for team in batched(["Ada", "Linus", "Grace", "Alan", "Hedy"], 2):
    print(team)

('Ada', 'Linus')
('Grace', 'Alan')
('Hedy',)

('Hedy',) is a tuple with one item. The comma is what makes it a tuple.