calc4.py

INTEGER, MUL, DIV, EOF = 'INTEGER', 'MUL', 'DIV', 'EOF'


class Token(object):
    def __init__(self, type, value):
        self.type = type
        self.value = value


    def __str__(self):
        return "Token({type}, {value})".format(
            type=self.type,
            value=repr(self.value)
        )


    def __repr__(self):
        return self.__str__()


class Lexer(object):
    def __init__(self, text):
        
        self.text = text
        self.pos = 0
        self.current_char = self.text[self.pos]


    def error(self):
        raise Exception("Invalid character")
    

    def advance(self):
        self.pos += 1
        if self.pos > len(self.text) - 1:
            self.current_char = None 
        else:
            self.current_char = self.text[self.pos]
    

    def skip_whitespace(self):
        while self.current_char is not None and self.current_char.isspace():
            self.advance()
        

    def integer(self):
        result = ""
        while self.current_char is not None and self.current_char.isdigit():
            result += self.current_char
            self.advance()
        return int(result)


    def get_next_token(self):

        while self.current_char is not None:

            if self.current_char.isspace():
                self.skip_whitespace()
                continue
            
            if self.current_char.isdigit():
                return Token(INTEGER, self.integer())
            
            if self.current_char == "*":
                self.advance()
                return Token(MUL, "*")
            
            if self.current_char == "/":
                self.advance()
                return Token(DIV, "/")
            
            self.error
        
        return Token(EOF, None)


class Interpreter(object):
    def __init__(self, lexer):
        self.lexer = lexer
        self.current_token = self.lexer.get_next_token()

    def error(self):
        raise Exception("INVALID SYNTAX")
    
    def eat(self, token_type):

        if self.current_token.type == token_type:
            self.current_token = self.lexer.get_next_token()
        else:
            self.error()

    def factor(self):
        token = self.current_token
        self.eat(INTEGER)
        return token.value

    def expr(self):
        result = self.factor()

        while self.current_token.type in (MUL, DIV):
            token = self.current_token
            if token.type == MUL:
               self.eat(MUL)
               result = result * self.factor()
            elif token.type == DIV:
                self.eat(DIV)
                result = result / self.factor()
        return result

def main():
    while True:
        try:
            text = input("Calc>>> ")
        except EOFError:
            break
        if not text:
            continue
        lexer = Lexer(text)
        interpreter = Interpreter(lexer)
        result = interpreter.expr()
        print(result)

if __name__ == "__main__":
    main()