In the Python solution, when we push a tuple of the form (number, number_index, list), we are actually storing a reference to the list into the heap. Alternatively, we could save the index of each list. For example:

    for i, a_list in enumerate(lists):
      heappush(min_heap, (a_list[0], 0, i))

    count = 0

    while min_heap:
      number, number_idx, list_idx = heappop(min_heap)
      count += 1

      if count == k:
        return number
      
      curr_list = lists[list_idx]
      if len(curr_list) > number_idx + 1:
        heappush(min_heap, (curr_list[number_idx + 1], number_idx + 1, list_idx))