用于将中缀字符串转换为二叉树的代码，我收到:无法编译的源代码-错误的sym类型

class Node:
    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None

def is_operator(char):
    operators = ['+', '-', '*', '/']
    return char in operators

def construct_expression_tree(expression):
    stack = []
    for char in expression:
        if is_operator(char):
            # 创建一个新的节点，并将栈顶的两个节点作为其左右子节点
            node = Node(char)
            node.right = stack.pop()
            node.left = stack.pop()
            stack.append(node)
        else:
            # 创建一个新的节点，并将其压入栈中
            node = Node(char)
            stack.append(node)
    # 栈中最后剩下的节点即为根节点
    return stack.pop()

def inorder_traversal(node):
    if node:
        inorder_traversal(node.left)
        print(node.value, end=" ")
        inorder_traversal(node.right)

# 测试代码
expression = "a+b*c"
root = construct_expression_tree(expression)
print("中缀表达式:", expression)
print("中序遍历结果:", end=" ")
inorder_traversal(root)