merge_lists.py

from heapq import merge

def merge_lists(lists):
    if not lists:
        return []
    while len(lists) > 1:
        a=[]
        new_lists = []
        if len(lists) % 2:
            new_lists.append(lists.pop())
        for a, b in zip(lists[::2], lists[1::2]):
            new_lists.append(merge(a, b))
        lists = new_lists
    return lists[0]