Get parsed result.

        After :func:`parse` the argv, we can get the parsed results::

            # command.option('-f', 'description of -f')
            command.get('-f')

            command.get('verbose')
            # we can also get ``verbose``: command.verbose

def get(self, key):
        """
        Get parsed result.

        After :func:`parse` the argv, we can get the parsed results::

            # command.option('-f', 'description of -f')
            command.get('-f')

            command.get('verbose')
            # we can also get ``verbose``: command.verbose
        """

        value = self._results.get(key)
        if value is not None:
            return value

        # get from option default value
        option = list(filter(lambda o: o.key == key, self._option_list))
        if not option:
            raise ValueError('No such option: %s' % key)

        option = option[0]
        return option.default