def set_coursera_repo_root_dir(self, coursera_repo_root_dir=None):
   self.coursera_repo_root_dir = coursera_repo_root_dir
   if self.coursera_repo_root_dir == None:
     self.coursera_repo_root_dir = ls.get_coursera_external_disk_rootdir_abspath()
   if not os.path.isdir(self.coursera_repo_root_dir):
     raise CourserasRepoRootFolderHasNotBeenFoundOrInexists, CourserasRepoRootFolderHasNotBeenFoundOrInexists.show_error_msg(self.external_disk_root_dir)  
 def init_constants(self):
   self.COURSERA_COURSES_XML_DATA_FILENAME = 'coursera_courses_folder_names.xml'
   self.COURSES_BASE_DIR_ON_EXTERNAL_DISK_ABSPATH = ls.get_coursera_external_disk_rootdir_abspath()