def demo_app(environ,start_response):
    # M: StringIO reads and writes a string buffer (also known as memory files).

    from StringIO import StringIO
    stdout = StringIO()
    print >> stdout, "Hello world!"
    print >> stdout

    h = environ.items()
    h.sort()
    for k,v in h:
        print >> stdout, k,‘=‘,`v`

    start_response("200 OK", [(‘Content-Type‘,‘text/plain‘)])

    return [stdout.getvalue()]

def make_server(
    host, port, app, server_class=WSGIServer, handler_class=WSGIRequestHandler
):
    """Create a new WSGI server listening on `host` and `port` for `app`"""

    # M: -> HTTPServer.__init__
    #    -> TCPServer.__init__
    #       -> TCPServer.server_bind
    #           -> TCPServer.socket.bind
    #       -> TCPServer.server_activate
    #           -> TCPServer.socket.listen
    server = server_class((host, port), handler_class)

    # M: conresponding to WSGIRequestHandler.handle()
    #    -> handler.run(self.server.get_app())
    server.set_app(app)

    return server

    httpd = make_server(‘‘, 8000, demo_app)
    sa = httpd.socket.getsockname()
    print "Serving HTTP on", sa[0], "port", sa[1], "..."

    # M: webbrowser provides a high-level interface to allow displaying Web-based documents 
    # to users. Under most circumstances
    import webbrowser
    webbrowser.open(‘http://localhost:8000/xyz?abc‘)

    httpd.handle_request()  # serve one request, then exit

def run(self, application):
        """Invoke the application"""
        # Note to self: don‘t move the close()!  Asynchronous servers shouldn‘t
        # call close() from finish_response(), so if you close() anywhere but
        # the double-error branch here, you‘ll break asynchronous servers by
        # prematurely closing.  Async servers must return from ‘run()‘ without
        # closing if there might still be output to iterate over.
        try:
            self.setup_environ()
            self.result = application(self.environ, self.start_response)
            self.finish_response()
        except:
            try:
                self.handle_error()
            except:
                # If we get an error handling an error, just give up already!
                self.close()
                raise   # ...and let the actual server figure it out.

    def start_response(self, status, headers,exc_info=None):
        """‘start_response()‘ callable as specified by PEP 333"""

        # M:
        # exc_info:
        #    The exc_info argument, if supplied, must be a Python sys.exc_info()
        #    tuple. This argument should be supplied by the application only if
        #    start_response is being called by an error handler.

        #    exc_info is the most recent exception catch in except clause

        #    in error_output:
        #        start_response(
        #             self.error_status,self.error_headers[:],sys.exc_info())

        # headers_sent:
        #    when send_headers is invoked, headers_sent = True
        #    when close is invoked, headers_sent = False

        if exc_info:
            try:
                if self.headers_sent:
                    # Re-raise original exception if headers sent
                    raise exc_info[0], exc_info[1], exc_info[2]
            finally:
                exc_info = None        # avoid dangling circular ref
        elif self.headers is not None:
            raise AssertionError("Headers already set!")

        assert type(status) is StringType,"Status must be a string"
        assert len(status)>=4,"Status must be at least 4 characters"
        assert int(status[:3]),"Status message must begin w/3-digit code"
        assert status[3]==" ", "Status message must have a space after code"
        if __debug__:
            for name,val in headers:
                assert type(name) is StringType,"Header names must be strings"
                assert type(val) is StringType,"Header values must be strings"
                assert not is_hop_by_hop(name),"Hop-by-hop headers not allowed"

        # M: set status and headers

        self.status = status

        # M:
        #    headers_class is Headers in module headers
        self.headers = self.headers_class(headers)

        return self.write

    def finish_response(self):
        """Send any iterable data, then close self and the iterable

        Subclasses intended for use in asynchronous servers will
        want to redefine this method, such that it sets up callbacks
        in the event loop to iterate over the data, and to call
        ‘self.close()‘ once the response is finished.
        """

        # M:
        #    result_is_file: 
        #       True if ‘self.result‘ is an instance of ‘self.wsgi_file_wrapper‘
        #    finish_content:
        #       Ensure headers and content have both been sent
        #    close:
        #       Close the iterable (if needed) and reset all instance vars
        if not self.result_is_file() or not self.sendfile():
            for data in self.result:
                self.write(data) # send data by self.write
            self.finish_content()
        self.close()

    def handle_error(self):
        """Log current error, and send error output to client if possible"""
        self.log_exception(sys.exc_info())
        if not self.headers_sent:
            self.result = self.error_output(self.environ, self.start_response)
            self.finish_response()
        # XXX else: attempt advanced recovery techniques for HTML or text?

    def error_output(self, environ, start_response):
        """WSGI mini-app to create error output

        By default, this just uses the ‘error_status‘, ‘error_headers‘,
        and ‘error_body‘ attributes to generate an output page.  It can
        be overridden in a subclass to dynamically generate diagnostics,
        choose an appropriate message for the user‘s preferred language, etc.

        Note, however, that it‘s not recommended from a security perspective to
        spit out diagnostics to any old user; ideally, you should have to do
        something special to enable diagnostic output, which is why we don‘t
        include any here!
        """

        # M:
        # sys.exc_info():
        #    Return information about the most recent exception caught by an except
        #    clause in the current stack frame or in an older stack frame.

        start_response(self.error_status,self.error_headers[:],sys.exc_info())
        return [self.error_body]