Your __call__()
should return wsgi-compliant result.
See http://www.python.org/dev/peps/pep-0333/ You can find following code sample:
def simple_app(environ, start_response):
"""Simplest possible application object"""
status = '200 OK'
response_headers = [('Content-type', 'text/plain')]
start_response(status, response_headers)
return ['Hello world!\n']