python 二叉树计算器

例子：计算1+2+3+4的值

代码：

class Buffer(object):
    """字符串处理函数"""

    def __init__(self, str_value):
        self._str_value = str_value
        self._off_set = 

    def peek(self):
        if self._off_set >= len(self._str_value):
            return None
        else:
            return self._str_value[self._off_set]

    def advance(self):
        self._off_set += 

class Token(object):
    """定义节点类型，｛int,2｝{ope,+}"""

    def consum(self, buffer):
        pass

class TokenInt(Token):
    """整数节点类型｛int,2｝"""

    def consum(self, buffer):
        accu = ""
        while True:
            ch = buffer.peek()
            if ch is None or ch not in "":
                break
            else:
                accu += ch
                buffer.advance()
        if accu != "":
            return "int", int(accu)
        else:
            return None

class TokenOperator(Token):
    """操作符接点类型，返回｛ope，+｝"""

    def consum(self, buffer):
        ch = buffer.peek()
        if ch is not None and ch in "+-":
            buffer.advance()
            return "ope", ch
        else:
            return None

class Node(object):
    """节点"""
    pass

class NodeInt(Node):
    """整数节点"""

    def __init__(self, value):
        self.value = value

class NodeOpe(Node):
    """加减节点"""

    def __init__(self, kind):
        self.kind = kind
        self.left = None
        self.right = None

def get_tokens(string):
    """根据string获取节点类型数组"""
    buffer = Buffer(string)
    tk_int = TokenInt()
    tk_ope = TokenOperator()
    tokens = []

    while buffer.peek():
        token = None
        for tk in (tk_int, tk_ope):
            token = tk.consum(buffer)
            if token:
                tokens.append(token)
                break
        if not token:
            raise ValueError("Error in syntax")
    return tokens

def parse(tokens):
    """将tokens生成二叉树"""
    # 判断第一个是不是数字
    if tokens[][] != 'int':
        raise ValueError('Error in syntax')
    # 新建第一个节点
    node = NodeInt(tokens[][])
    # 下一个节点
    node_next = None
    # 节点类型
    node_type = tokens[][]
    for token in tokens[:]:
        # 判断节点类型是否一致
        if token[] == node_type:
            raise ValueError("error in syntax")
        node_type = token[]
        # 判断是什么操作符
        # 如果是符号
        if token[] == 'ope':
            node_next = NodeOpe(token[])
            node_next.left = node
        if token[] == 'int':
            node_next.right = NodeInt(token[])
            node = node_next
    return node

def calculate(node):
    """迭代求值"""
    if isinstance(node.left, NodeOpe):
        left_value = calculate(node.left)
    else:
        left_value = node.left.value

    if node.kind == '-':
        return left_value - node.right.value
    elif node.kind == '+':
        return left_value + node.right.value
    else:
        raise ValueError('Error in syntax')

def evalute(node):
    """判断是否只有一个数值"""
    if isinstance(node, NodeInt):
        return node.value
    else:
        return calculate(node)

if __name__ == '__main__':
    input_str = input("input:")
    tokens = get_tokens(input_str)
    node = parse(tokens)
    print("value is {}".format(evalute(node)))