Swapping two sublists in a list

Question

Given the following list:

my_list=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

I want to be able to swap the sub-list my_list[2:4] with the sub-list my_list[7:10] as quickly and as efficiently as possible, to get the new list:

new_list=[0, 1, 7, 8, 9, 4, 5, 6, 2, 3, 10, 11, 12]

Here's my attempt:

def swap(s1, s2, l):

    seg1=l[:s1.start]+l[s2]
    seg2=l[s1.stop : s2.start]
    seg3=l[s1]+l[s2.stop:]

    return seg1+seg2+seg3


print swap(slice(2,4), slice(7,10), [0,1,2,3,4,5,6,7,8,9,10,11,12])

This does print the desired output, although this way of doing it looks awful to me.

Is there a more easy and elegant way of doing it, that will not create four new lists for every function call? (I plan to call this function a lot)

I don't mind (actually I'd prefer) changing the original list, rather than creating new instance every function call.

Answer 1

Slices can be assigned.

Two variables can be swapped with a, b = b, a.

Combine the two above::

>>> my_list[7:10], my_list[2:4] = my_list[2:4], my_list[7:10]
>>> my_list
[0, 1, 7, 8, 9, 4, 5, 6, 2, 3, 10, 11, 12]

---

_{Beware that - if slices have different sizes - the order is important: If you swap in the opposite order, you end up with a different result, because it will change first the initial items (lower indices), and then the higher index items (but those will be shifted in a different position by the first assignment).}

_{Also, slices must not overlap.}