This code:

    assert type(val) is StringType,"Header values must be strings"

(from here http://svn.python.org/view/python/tags/r271/Lib/wsgiref/handlers.py?revision=86833&view=markup)

from "start_response" method, is not allowing to use str subclasses objects as header value.

Usecase:

I made class URL which subclasses str and has additional methods to manipulate query string. It is very handy. But when I need to set header
"Location" with URL object as value I get assertion error.

Can't we do this instead:

    assert isinstance(val, str),"Header values must be strings"