python str object does not support item assignment

I'm trying to set the scrambled word from the list back to the list I have created, which is from split. I tried reading some of the solutions here and I think it's because you can't change the string in the list?

I'm not really sure correct me if I'm wrong :( . the sentence[i] = temp_word is giving the error. thanks in advance :)

class WordScramble:
def __init__(self):
    self.user_input = input("Please give me a sentence: ")

def scramble(self):
    # print what was input
    print("The user input was: ", self.user_input)

    # first scramble is just one word
    print(self.user_input[0] + self.user_input[2] + self.user_input[1] + self.user_input[3:])

    # reverse two indices
    # particularly good to use is to switch the first two
    # and the last two
    # this only makes sense if you have a world that is longer than 3


    # now try to scramble one sentence
    sentence = self.user_input.strip().split(" ")

    for i, word in enumerate(sentence):
        if len(word) > 3:
            temp_word = list(word)
            if ',' in temp_word:
                temp = temp_word[1]
                temp_word[1] = temp_word[-3]
                temp_word[-3] = temp

            else:
                temp = temp_word[1]
                temp_word[1] = temp_word[2]
                temp_word[2] = temp

            temp_word = ''.join(temp_word)
            sentence[i] = temp_word
        sentence = ''.join(sentence)
        print(sentence)




    #print(" ".join(sentence))

    # do just words first, then you can move on to work on
    # punctuation

word_scrambler = WordScramble()
word_scrambler.scramble()

Because inside the for loop you wrote:

sentence = ''.join(sentence)

Thus, at the second iteration, the 'sentence' variable is now a string and in python, strings don't support item assignment as they are immutable variables. I think you meant to get this out of the for loop to print the final sentence.