@classmethod
def _add_served_directory(cls, config, relative_path, config_var):
    """ Add extra public/template directories to config. """
    import inspect
    import os
    assert config_var in ('extra_template_paths', 'extra_public_paths')
    frame, filename, line_number, function_name, lines, index = (inspect.
        getouterframes(inspect.currentframe())[2])
    this_dir = os.path.dirname(filename)
    absolute_path = os.path.join(this_dir, relative_path)
    if absolute_path not in config.get(config_var, ''):
        if config.get(config_var):
            config[config_var] += ',' + absolute_path
        else:
            config[config_var] = absolute_path
