Compressing And Decompressing into .txt in python

So i want to create a program that asks the user to compress or decompress and if compress it asks for a sentence and compresses the outcome and if decompress it saves the outcome of that in a .txt too. I have this currently.

import pickle


compressopen = input("Do you want to compress or uncompress: ")
compressopen.lower()

if compressopen == "compress":

    sentence = input("Please enter a sentence: ")
    sentencesplit = sentence.split()

    print(sentencesplit)
    listnew = []

    for x in sentencesplit:
        if x not in listnew:

            listnew.append(x)
    print(listnew)

    postion = []


    for c in sentencesplit:
        (postion.append(listnew.index(c)+1))

elif compressopen == "uncompress":