calc2.py

EOF, PLUS, MINUS,INTEGER = 'EOF', 'PLUS', 'MINUS','INTEGER'

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 Interpreter(object):
    def __init__(self, text):
        self.text = text
        self.pos = 0
        self.current_token = None
        self.current_char = self.text[self.pos]

    def error(self):
        return Exception("Error parsing input")

    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:

            # 先处理空格逻辑
            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(PLUS,'+')

            if self.current_char == '-':
                self.advance()
                return Token(MINUS, '-')

            self.error()

        return Token(EOF, None)

    def eat(self, token_type):
        if self.current_token.type == token_type:
            self.current_token = self.get_next_token()
        else:
            self.error()

    def expr(self):
        self.current_token = self.get_next_token()

        left = self.current_token
        self.eat(INTEGER)

        op = self.current_token
        if op.type == PLUS:
            self.eat(PLUS)
        else:
            self.eat(MINUS)

        right = self.current_token
        self.eat(INTEGER)

        if op.type == PLUS:
            result = left.value + right.value
        elif op.type == MINUS:
            result = left.value - right.value

        return result

def main():
    while True:
        text = input("calc2 > ")
        interpreter = Interpreter(text)
        result = interpreter.expr()
        print(result)

if __name__ == "__main__":
    main()