使用Python和lxm将代码写入元素属性

Question

我想使用lxml/xpath查找某些img元素，并在它们的src属性中编写一个简短的php脚本。像这样：

from lxml import html

htmldoc = html.document_fromstring(htmlstr)
imgs = htmldoc.xpath("//*[@class='someclass']/img")
imgs[0].attrib['src'] = "<?php echo get_img_file(); ?>"
processedHTML = html.tostring(htmldoc, pretty_print=True)

with open("test.php","w+") as outfile:
    outfile.write(processedHTML.decode("utf-8"))

非法字符（如<；和>；）将作为html实体转义。有没有办法设置lxml以允许将这些字符写入文档？谢谢！你知道吗