Adding (user) callback to normalize option value ?

Here is my usecase: I create a tornado HTTPServer and its Application. They are configured with options both from parse_command_line and parse_config_file. For instance I define the option:

options.define(
    'a-file-to-load',
    type=str,
    metavar='path',
    help='This file will be loaded later in the application')

I parse my config, initialize the app, start the HTTPServer; everything is going well. Then when everything is OK I daemonize my process. One of the operation is to change the current working directory to /. But my a-file-to-load option is maybe a relative path. When I try to open it later, it fails.

My concern is here to normalize my option value after parsing. I did try to put a callback:

def normalize_callback(path):
    options.a_file_to_load = os.path.abspath(path)

But it loops recursively on the callback (because _Option#set method is actually calling the callback as well).

My question is: can we add a callback to normalize the option value or can we pass a flag to callback to avoid recursion ? Otherwise do you have any good idea to handle this use case ?

Start by reading the _Option#set method and the paths used by options.define, parse_command_line, and parse_config_file. Trace how callbacks are invoked and how option values are assigned; done requires a decided, non-recursive normalization API that covers the described daemonization case and has its behavior verified.