XML如何使用SAX修改内容

<bookstore>
  <book>
    <title>The Lord of the Rings</title>
    <price>29.99</price>
  </book>
  <book>
    <title>The Hitchhiker's Guide to the Galaxy</title>
    <price>12.99</price>
  </book>
</bookstore>

import xml.sax
import xml.sax.saxutils

class BookHandler(xml.sax.ContentHandler):
    def __init__(self, output_file):
        self.output_file = output_file
        self.in_book = False
        self.in_price = False
        self.current_title = ""
        self.current_price = ""

    def startElement(self, name, attrs):
        if name == "book":
            self.in_book = True
        elif name == "price" and self.in_book:
            self.in_price = True

    def characters(self, content):
        if self.in_price:
            self.current_price = content.strip()

    def endElement(self, name):
        if name == "book":
            self.in_book = False
            self.output_file.write(f'<book><title>{self.current_title}</title><price>39.99</price></book>\n') #修改价格并写入
            self.current_title = ""
            self.current_price = ""
        elif name == "price":
            self.in_price = False
        elif name == "title":
            self.current_title = self.current_price #此处是错误的，应该直接读取title

def modify_xml(input_file, output_file):
    parser = xml.sax.make_parser()
    handler = BookHandler(output_file)
    parser.setContentHandler(handler)
    parser.parse(input_file)

# 使用示例
input_file = "bookstore.xml"
output_file = open("bookstore_modified.xml", "w")
output_file.write('<bookstore>\n') #添加bookstore标签
modify_xml(input_file, output_file)
output_file.write('</bookstore>') #添加bookstore结束标签
output_file.close()