immutable_func.py

from functools import cmp_to_key
def sort_cmp(elem1, elem2):
	if len(elem1) > len(elem2):
		return -1
	elif len(elem1) < len(elem2):
		return 1
	else:
		if elem1 > elem2:
			return -1
		else:
			return 1
		
def immutable_list(string_list, add_list, remove_list):
	for elem in add_list:
		if elem not in string_list:
			string_list.append(elem)
	
	for elem in remove_list:
		if elem in string_list:
			# can safely use this as elements are unique in list
			string_list.remove(elem)
	
	# sort according to follwing rules
	"""
	List shall be ordered as follows
	--- Most character count to least character count
	--- In the event of a tie, reverse alphabetical
	"""
	string_list = sorted(string_list, key=cmp_to_key(sort_cmp))
	return string_list
	
print(immutable_list(["one","two", "three"], ["one","two","five","six"], ["two","five"]))