Generate a unique (absolute) file path within the file_dir directory

        :rtype: String
        :returns: a unique file path

def __generate_filepath(self):
        """ Generate a unique (absolute) file path within the file_dir directory

        :rtype: String
        :returns: a unique file path


        """
        file_path = os.path.join(self.file_dir, str(uuid.uuid4()))
        if os.path.exists(file_path):
            warnings.warn("something has almost certainly gone wrong")
            return self.__generate_filepath()
        return file_path