def add_plugin_directories(self, paths, except_blacklisted=True):
        """
        Adds `directories` to the set of plugin directories.

        `directories` may be either a single object or a iterable.

        `directories` can be relative paths, but will be converted into
        absolute paths based on the current working directory.

        if `except_blacklisted` is `True` all `directories` in
        that are blacklisted will be removed
        """
        self.directory_manager.add_directories(paths, except_blacklisted)