Why does the slice operation behave inconsistently in my Python code?

The slice operation does not seem consistent. In some cases slice includes the kth value and sometimes it doesn’t.

rotate.py

def rotate(nums, k):
    temp = nums[0:k+1]
    del nums[0:k+1]
    nums.extend(temp)

test_rotate.py

import unittest
from rotate_array import rotate

class TestRotateArray(unittest.TestCase):
	def test_rotate_array2(self):
		nums= [-1, -100, 3, 99]
		k = 2
		expected = [3, 99, -1, -100]
		rotate(nums, k)
		self.assertListEqual(nums, expected)
	
	def test_rotate_array(self):
		nums= [1,2,3,4,5,6,7]
		k = 3
		expected = [5,6,7,1,2,3,4]
		rotate(nums, k)
		self.assertListEqual(nums, expected)

if __name__ == '__main__':
		unittest.main()

The first test will fail and the second will pass when I do nums[:k+1].
the nums array for the failing test: [99, -1, -100, 3]

When I use nums[0:k] the first one passes and the second does not.
the nums array for the failing test: [4, 5, 6, 7, 1, 2, 3]

The whole question I am working on can be found here

You are missing an additional index. Since you are stopping at i+1, you will slice three elements if you have k = 2 --> (0, 1, 2, except the 3).

In your first case temp = [-1, -100, 3] and the num will be [99, -1, -100, 3]

I modified your code a little bit to test my hypothesis.

def rotate(nums, k):
    temp = nums[0:k+1]
    del nums[0:k+1]
    nums.extend(temp)

nums= [-1, -100, 3, 99]
k = 2
expected = [3, 99, -1, -100]
rotate(nums, k)
print("One:", nums, expected)


nums= [1,2,3,4,5,6,7,8]
k = 4
expected = [5,6,7,8,1,2,3,4]
rotate(nums, k)

print("Two:", nums, expected)

I assume you are trying to rotate the list from the middle. The mid-point of the list is different when the length of the list is an odd number in which case you have a single middle point, thus you refer the middle point using [:k+1] correctly.

In case of a list with even number of elements there are two middle points, example a list=[1,2,3,4,5,6,7,8], there is no single middle point but two of them (4 and 5). Now, the question comes which one to consider for rotation, in your case, you want to consider the first one and thus [0:k] works. Note- [:k+1] will take the second middle point.

The solution would be to check if the list has an odd or even length and apply slicing accordingly. I hope this should work-

def rotate(nums, k):
    if len(nums)%2 == 1:
        temp = nums[0:k+1]
        del nums[0:k+1]
    else:
        temp = nums[0:k]
        del nums[0:k]
    
    nums.extend(temp)

Accordingly your implementation of rotate the first test does not pass because you are pushing back the first three elements [0:k+1] instead you want to push the last two numbers in the top of the list so:

def rotate(nums, k):
  return nums[-k:] + nums[0:-k]

In this way you must remember to re-assign nums = rotate(nums, k). If you want to make methods with side effects, remember to put _ at the end of the methods as a typical Python convention.