lxml etree.iterparse错误"TypeError:读取文件对象必须返回纯字符串"

import lxml.etree as ET

with open('file.xml', 'r', encoding='utf-8') as f:
    for event, element in ET.iterparse(f):
        # 处理事件
        pass

import lxml.etree as ET
import chardet

with open('file.xml', 'rb') as f:
    result = chardet.detect(f.read())
    encoding = result['encoding']

with open('file.xml', 'r', encoding=encoding) as f:
    for event, element in ET.iterparse(f):
        # 处理事件
        pass