scanner.py

import string


class SyntaxError(Exception):
    pass           # local errors


class LexicalError(Exception):
    pass          # used to be strings


class Scanner:
    def __init__(self, text):
        self.next = 0
        self.text = text + '\0'

    def new_text(self, text):
        Scanner.__init__(self, text)

    def showerror(self):
        print('=> ', self.text)
        print('=> ', (' ' * self.start) + '^')

    def match(self, token):
        if self.token != token:
            raise SyntaxError(token)
        else:
            value = self.value
            if self.token != '\0':
                self.scan()                  # next token/value
            return value                     # return prior value

    def scan(self):
        self.value = None
        ix = self.next
        while self.text[ix] in string.whitespace:
            ix += 1
        self.start = ix

        if self.text[ix] in ['(', ')', '-', '+', '/', '*', '^', '\0']:
            self.token = self.text[ix]
            ix += 1

        elif self.text[ix] in string.digits:
            str = ''
            while self.text[ix] in string.digits:
                str += self.text[ix]
                ix += 1
            if self.text[ix] == '.':
                str += '.'
                ix += 1
                while self.text[ix] in string.digits:
                    str += self.text[ix]
                    ix += 1
                self.token = 'num'
                self.value = float(str)
            else:
                self.token = 'num'
                self.value = int(str)           # subsumes long() in 3.x

        elif self.text[ix] in string.ascii_letters:
            str = ''
            while self.text[ix] in (string.digits + string.ascii_letters):
                str += self.text[ix]
                ix += 1
            if str.lower() == 'set':
                self.token = 'set'
            else:
                self.token = 'var'
                self.value = str

        else:
            raise LexicalError()
        self.next = ix