Rotate a list in Python

To rotate a list left by 2 places, move the first 2 items to the end: [1, 2, 3, 4, 5] becomes [3, 4, 5, 1, 2].

The program

def rotate_left(items, k):
    if not items:
        return []
    k = k % len(items)
    return items[k:] + items[:k]

numbers = [1, 2, 3, 4, 5]

print(rotate_left(numbers, 2))
print(rotate_left(numbers, 7))
print(rotate_left(numbers, -1))
print(numbers)

[3, 4, 5, 1, 2]
[3, 4, 5, 1, 2]
[5, 1, 2, 3, 4]
[1, 2, 3, 4, 5]

How it works

• items[k:] is everything from position k to the end, and items[:k] is the first k items. Joining them in that order gives the rotated list.
• Rotating a list of 5 items by 5 places gives the same list again. So k % len(items) removes the full turns: rotating by 7 is the same as rotating by 2.
• With a negative k, % gives a positive number in Python: -1 % 5 is 4. Rotating left by 4 is the same as rotating right by 1.
• The function builds a new list, so numbers does not change.
• The empty-list check stops % 0, which would raise a ZeroDivisionError.

Another way: deque.rotate()

A deque from the collections module has a rotate() method. It changes the deque in place, and a positive number rotates to the right:

from collections import deque

d = deque([1, 2, 3, 4, 5])
d.rotate(2)
print(list(d))
d.rotate(-2)
print(list(d))

[4, 5, 1, 2, 3]
[1, 2, 3, 4, 5]