Boolean won't change in Python

Question

I am attempting to create a program in Python that asks a user to input a string (preferably in lower-case), and then convert that string into sentence case. However, a boolean I am using in order to check whether the next letter needs to be capitalised will not be set to False, despite the conditions required for the 'if' statement being met.

    class SentenceCaseProgram(object):
        def __init__(self, isPunctuation, isSpace, sentence, new_sentence):
            self.isPunctuation = isPunctuation
            self.sentence = sentence
            self.new_sentence = new_sentence
            self.isSpace = isSpace
            self.count = 0
        def Input(self):
            self.sentence = str(input("Type in a sentence (with punctuation) entirely in lowercase. "))
        def SentenceCase(self):
            for letter in self.sentence:
                print(self.isPunctuation)
                if self.count == 0:
                    letter = letter.capitalize()
                if letter is ' ':
                    self.isSpace = True
                if (self.isPunctuation == True) and (letter in 'abcdefghijklmnopqrstuvwxyz'):
                    letter = letter.capitalize()
                    self.isPunctuation = letter is '.' or '!' or '?' or ')'
                if letter is 'i' and self.isSpace is True:
                    letter = letter.capitalize()
                    self.isSpace = False
                self.count += 1
                if letter == '.' or '!' or '?' or ')':
                    self.isPunctuation = True
                else:
                    self.isPunctuation = False
                self.new_sentence += letter
        def Print(self):
            print("Your sentence in sentence case is '%s'" % self.new_sentence)
        def Main(self):
            self.__init__(False, False, "", "")
            self.Input()
            self.SentenceCase()
            self.Print()
    app = SentenceCaseProgram(False, False, "", "")
    app.Main()

When I run the program, the program asks for an input, and then capitalizes every single letter in the sentence, and the self.isPunctuation boolean will constantly be set to True, apart from with the first loop.

Answer 1

don't use is to compare strings, use ==

letter is '.' or '!' or '?' or ')' should be

letter == '.' or letter == '!' or letter == '?' or letter == ')'

OR

letter in '.!?)'