python 一遍式四则运算

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#!/usr/bin/python #coding: utf-8   INTEGER = 'INTEGER' PLUS    = '+' MINUS   = '-' MUL     = '*' DIV     = '/' LC      = '(' RC      = ')' EOF     = 'EOF'   class Token(object):     def __init__(self, typ, val):         self.typ = typ;         self.val = val;       def __str__(self):         return "Token('{typ}','{val}')".format(typ=self.typ, val=self.val)       def __repr__(self):         return self.__str__()   class Lexer(object):     def __init__(self, text):         self.text = text         self.pos = 0         self.cur_token = None;         self.status = 0       def next_token(self):         if self.pos > len(self.text) -1:             return Token(EOF, '')           ch = ''         while True:             ch = self.text[self.pos]             self.pos+=1               if (ch!=' ' and ch!='\t'):                 break;           if self.status==0:             if ch.isdigit():                 self.status = 1                 return Token(INTEGER, int(ch + self.next_token().val))             else:                 return Token(ch, ch)         elif self.status==1:             if ch.isdigit():                 return Token(INTEGER, ch + self.next_token().val)             else:                 self.pos -= 1                 self.status = 0                 return Token(INTEGER, '')               class Interpreter(object):     def __init__(self, lexer):         self.lexer = lexer         self.cur_token = lexer.next_token()       def error(self):         raise Exception("syntax error")       def eat(self, typ):         tok = self.cur_token         if (tok.typ == typ):             self.cur_token = self.lexer.next_token()             return tok.val         else:             self.error()       def factor(self):         if (self.cur_token.typ == INTEGER):             tok = self.cur_token             self.eat(INTEGER)             return tok.val         elif self.cur_token.typ == LC:             self.eat(LC)             result = self.expr()             self.eat(RC)             return result         else:             self.error()       def term(self):         result = self.factor()                   while self.cur_token.typ in (MUL, DIV):             if (self.cur_token.typ==MUL):                 self.eat(MUL)                 result = result * self.factor()             elif (self.cur_token.typ==DIV):                 self.eat(DIV)                 result = result / self.factor()           return result       def expr(self):         result = self.term()                   while self.cur_token.typ in (PLUS, MINUS):             if (self.cur_token.typ==PLUS):                 self.eat(PLUS)                 result = result + self.term()             elif (self.cur_token.typ==MINUS):                 self.eat(MINUS)                 result = result - self.term()           return result     def main():     print("expr2")     while True:         text = raw_input("calc> ")         if not text:             continue           if text=="exit":             break;           lex = Lexer(text)         calc = Interpreter(lex)         result = calc.expr()         print(result)   if __name__ == "__main__":     main()