被官方参考误解：http://webpy.org/cookbook/custom_notfound.zh-cn
不要使用notfound()方法就是了。
# coding:utf-8
import web
urls = (
'/', 'index'
)
app = web.application(urls, globals())
render = web.template.render('templates/')
def my_notfound():
return web.notfound("""
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>pctools</title>
</head>
<body>
404
</body>
</html>
""")
web.webapi.notfound = my_notfound
app = web.application(urls, globals())
if __name__ == '__main__':
app.run()