Print tree structure of folder with indentation

tree.py

import os

def list_files(startpath):
    for root, dirs, files in os.walk(startpath):
        level = root.replace(startpath, '').count(os.sep)
        indent = ' ' * 4 * (level)
        yield ('{}{}/'.format(indent, os.path.basename(root)))
        subindent = ' ' * 4 * (level + 1)
        for f in files:
            yield ('{}{}'.format(subindent, f))

if __name__ == '__main__':
    films_folder = '/drives/e/Peliculas'
    films_file = 'pelis.txt'

    with open(films_file, 'w') as file_pelis:
        for file_listed in list_files(films_folder):
            # \n{}.format() prints newlines as expected
            file_pelis.write('\n{}'.format(file_listed))