Adding a custom export command to python-ly's ly.server

ly-server.py

#!/usr/bin/env python3
#
# python-ly server that adds an a custom export command
#
# tested with python-ly version 0.9.8
# https://github.com/frescobaldi/python-ly
#
import copy
from http.server import HTTPServer

from ly.server.main import parse_command_line
from ly.server.command import _export_command
import ly.server.handler as handler

class export_custom(_export_command):
    def export(self, opts, cursor, exports):
        # Yse cursor.document as Lilypond document to work with
        # here we return a dummy value, later serialized as JSON.
        return "Hi there"

class CustomRequestHandler(handler.RequestHandler):
    def create_command(self, cmd):
        if cmd.get('command') == 'customexport':
            # note that this doesn't handle variables or args
            return [export_custom()]
        else:
            return super().create_command(cmd)

def main():
    server_opts, cmd_opts = parse_command_line()
    handler.default_opts = copy.deepcopy(cmd_opts)
    exit_code = 0
    try:
        server = HTTPServer(('', server_opts.port), CustomRequestHandler)
        print("Welcome to the python-ly HTTP server, with custom export")
        print("Listening on port {port}".format(port=server_opts.port))
        server.serve_forever()
    except KeyboardInterrupt:
        print("\nKeyboard interrupt received. Shutting down server")
        server.socket.close()
        print("Successfully closed. Bye...")
    return exit_code 

if __name__ == '__main__':
    main()