`URLDispatch` sends incoming requests to applications based on
prefixes.  First, lets create an application that tells us about what
its ``SCRIPT_NAME`` and ``PATH_INFO`` are::

    >>> def path_app(environ, start_response):
    ...     start_response('200 OK', [('Content-type', 'text/plain')])
    ...     return [environ.get('SCRIPT_NAME', ''), ':',
    ...             environ.get('PATH_INFO', '')]

Then we create a list of these applications and create the
dispatcher::

    >>> from urldispatch import URLDispatch
    >>> app_list = [('/foo', path_app), ('/bar', path_app)]
    >>> dispatch = URLDispatch(app_list)

We should see the proper values, plus 404 Not Found when no prefix
matches::

    >>> from testrunner import send_request
    >>> print send_request(dispatch, '/foo/bar')
    200 OK
    ...
    /foo:/bar
    >>> print send_request(dispatch, '/bar/XXX')
    200 OK
    ...
    /bar:/XXX
    >>> print send_request(dispatch, '/baz')
    404 Not Found
    ...

If we want a default application we can use the empty string::

    >>> from simpleapp import simple_app
    >>> app_list.append(('', simple_app))
    >>> dispatch = URLDispatch(app_list)
    >>> print send_request(dispatch, '/baz')
    200 OK
    ...
    ...Simple App...
    ...