'Ensure that a configuration value is available for a path.
If necessary, prompts the user and stores the result.
:param str key: The configuration key.
:param str label: The user-friendly label for this piece of information.'

def _configure_file(self, key, label, validator=None):

    configured_value = self.conf(key)
    if (not configured_value):
        new_value = self._prompt_for_file(label, validator)
        setattr(self.config, self.dest(key), os.path.abspath(os.path.expanduser(new_value)))
