Tokeninzing lisp in python

lisptoken.py

#!/usr/bin/python3


class Tokenizer:
    OPEN_BRACKET = '('
    CLOSE_BRACKET = ')'
    """Tokenizer class is used to tokenize a lisp statement into separate symbols"""
    def __init__(self, expression):
        """Class constructor used to initialize some member values"""
        self.exp = expression.strip()
        self.offset = 0
        if self.exp[self.offset] is not self.OPEN_BRACKET:
            raise Exception("Unexpected character '" + self.exp[self.offset] + "' found at 0th index. Expected '" + self.OPEN_BRACKET + "'")

    def match_pair(self, starter, ender):
        """Returns the index of the matching pair for a given 2 characters"""
        moff = self.offset
        if self.exp[self.offset] is not starter:
            return -1
        count = 1
        self.offset += 1
        while self.offset < len(self.exp):
            if self.exp[self.offset] is starter:
                count += 1
            if self.exp[self.offset] is ender:
                count -= 1
                if count is 0:
                    xx = self.offset
                    self.offset = moff
                    return xx
            self.offset += 1
        self.offset = moff
        return -1

    def get_tokens(self):
        """Returns a list containing the tokens for the lisp statement."""
        tokens = []
        index = self.match_pair(self.OPEN_BRACKET, self.CLOSE_BRACKET)
        if index is -1:
            raise Exception("Bracket match not found")
        temp = ""
        while (self.offset + 1) < len(self.exp):
            self.offset += 1
            curoff = self.offset
            ch = self.exp[curoff]
            if ch is self.OPEN_BRACKET:
                mindex = self.match_pair(self.OPEN_BRACKET, self.CLOSE_BRACKET)
                tokens.append(self.exp[curoff:mindex+1])
                self.offset = mindex
                continue
            if ch is ' ':
                if temp.strip() is not "":
                    tokens.append(temp)
                    temp = ""
                continue
            if ch.strip() is '':
                if temp.strip() is not "":
                    tokens.append(temp)
                    temp = ""
                continue
            temp += ch
        return tokens

print(Tokenizer(input().strip()).get_tokens())