我正在尝试使用简单的堆栈推送和弹出来反转字符串。然而，我收到了一些我不能理解的错误

class Stack:
    def __init__(self):
        self.items = []

    def is_empty(self):
        return len(self.items) == 0

    def push(self, item):
        self.items.append(item)

    def pop(self):
        if not self.is_empty():
            return self.items.pop()
        else:
            raise IndexError("pop from empty stack")

def reverse_string(s):
    stack = Stack()
    for char in s:
        stack.push(char)
    
    reversed_string = ""
    while not stack.is_empty():
        reversed_string += stack.pop()
    
    return reversed_string

# 测试
input_string = "hello"
print(f"Original string: {input_string}")
print(f"Reversed string: {reverse_string(input_string)}")

def pop(self):
    if not self.is_empty():
        return self.items.pop()
    else:
        raise IndexError("pop from empty stack")

for char in s:
    stack.push(char)