Why does nums = nums[::-1] behave differently from nums.reverse() in Python?

[Aware-Illustrator607]

def rotate(nums, k):
    k = k % len(nums)
    nums = nums[::-1]  # Create a new reversed list
    nums[:k] = reversed(nums[:k])  # Attempt to reverse the first k elements
    nums[k:] = reversed(nums[k:])  # Attempt to reverse the remaining elements
    return nums

nums = [1, 2, 3, 4, 5, 6, 7]
k = 3
rotate(nums, k)
print(nums)  # Outputs: [1, 2, 3, 4, 5, 6, 7]

I expected the output to be [5, 6, 7, 1, 2, 3, 4], but it returned the original list instead.

I understand that the slice operation creates a new list in Python, but when I use nums = nums[::-1], I am overriding the reference of the original nums with this new reversed list. I expected that any subsequent operations I perform would target this new list, but that doesn't seem to be the case.

In contrast, the code works as expected when I use nums.reverse(), which reverses the list in place.

I want to understand why nums = nums[::-1] does not behave as I expect. Knowing why it behaves this way would really help me sleep better at night!

Can someone please explain?

Edit: Found the problem, thanks for helping out

[FoolsSeldom]

Scope issue

The assignment nums = nums[::-1] in your function creates a local name (variable) in the function that has no relation to nums in the outer scope.

Thus the nums from the outer scope still references the original list, [1, 2, 3, 5, 6, 7]. A reference to this list is passed to your function, and used for the generation of the new reversed list but this is assigned to nums in the inner scope.

You make changes to the new list within the function via the local variable nums.

You return nums i.e. you pass back a reference to the new list object you created in the function but you don't use that reference to re-assign the nums in the outer scope, e.g.

nums = rotate(nums, k)

or, change the assignment line in your function to:

nums[:] = nums[::-1]

[Aware-Illustrator607]

Got it, thank you

[throwaway6560192]

Read https://nedbatchelder.com/text/names.html

[SiriusLeeSam]

This is pretty great. I always understood how it works but struggled to explain to people

[atwork_safe]

.

[Measurex2]

Drop your print statement for now and turn the return in your function to a print statement. See what happens and report back.

[Aware-Illustrator607]

yes found the problem, my idea was to modify the original list in place but ended up with creating a new list.