The following code pops at the end (efficient enough) and sorts inplace despite returning as well.
def mergesort(lis):
if len(lis) > 1:
left, right = map(lambda l: list(reversed(mergesort(l))), (lis[::2], lis[1::2]))
lis.clear()
while left and right:
lis.append(left.pop() if left[-1] < right[-1] else right.pop())
lis.extend(left[::-1])
lis.extend(right[::-1])
return lis