python:get files in the directory which includes several subdirectories

get_files_in_the_directory.py

# Only one-level subfolders are supported
import os

def get_files(DIR):
    file_list=list()
    for sub_dir in os.listdir(DIR):
        if os.isdir(sub_dir):
            for file in os.listdir(os.path.join(DIR,sub_dir)):
                file_list.append(file)
    return file_list