Обе эти работы:
[f for (subdir, dirs, files) in os.walk(rootdir) for f in files]
sum([files for (subdir, dirs, files) in os.walk(rootdir)], [])
Пример вывода:
$ find /tmp/test
/tmp/test
/tmp/test/subdir1
/tmp/test/subdir1/file1
/tmp/test/subdir2
/tmp/test/subdir2/file2
$ python
>>> import os
>>> rootdir = "/tmp/test"
>>> [f for (subdir, dirs, files) in os.walk(rootdir) for f in files]
['file1', 'file2']
>>> sum([files for (subdir, dirs, files) in os.walk(rootdir)], [])
['file1', 'file2']