def currency_code(s):

    if not ((len(s) == 3) and s.isalpha()):

        raise ValueError("bad currency code: {!r}".format(s))

    return s.upper()

def currency_list(s):

    return [currency_code(code.strip()) for code in s.split(',')]

def date_from(fmt_s):

    def date_from_fmt(s):

        return datetime.datetime.strptime(s, fmt_s).date()

    return date_from_fmt

class Configuration:

    DEFAULT_CONFIG_PATH = pathlib.Path(HOME_PATH, '.config', 'oxrlib.ini')

    PREPOSITIONS = frozenset(['in', 'to', 'into'])

    def __init__(self, arglist):

        argparser = self._build_argparser()

        self.error = argparser.error

        self.args = argparser.parse_args(arglist)

        if self.args.config_file is None:

            self.args.config_file = [self.DEFAULT_CONFIG_PATH]

        self.conffile = self._build_conffile()

        conffile_paths = [path.as_posix() for path in self.args.config_file]

        read_files = self.conffile.read(conffile_paths)

        for expected_path, read_path in zip(conffile_paths, read_files):

            if read_path != expected_path:

                self.error("failed to read configuration file {!r}".format(expected_path))

        try:

            post_hook = getattr(self, '_post_hook_' + self.args.command)

        except AttributeError:

            pass

        else:

            post_hook()

    def _build_argparser(self):

        prog_parser = argparse.ArgumentParser()

        prog_parser.add_argument(

            '--config-file', '-c',

            action='append', type=pathlib.Path,

            help="Path of a configuration file to read",

        )

        subparsers = prog_parser.add_subparsers()

        hist_parser = subparsers.add_parser(

            'historical', aliases=['hist'],

            usage='%(prog)s YYYY-MM-DD [[amount] code] [[in] code]',

            help="Show a currency conversion or rate from a past date",

        )

        hist_parser.set_defaults(

            command='historical',

            amount=None,

            from_currency=None,

            ledger=None,

        )

        hist_parser.add_argument(

            '--base',

            metavar='CODE', type=currency_code,

            help="Base currency (default USD)",

        )