Another way of returning all files in subdirectories is to use the pathlib module, introduced in Python 3.4, which provides an object oriented approach to handling filesystem paths (Pathlib is also available on Python 2.7 via the pathlib2 module on PyPi):
from pathlib import Path
rootdir = Path('C:/Users/sid/Desktop/test')
# Return a list of regular files only, not directories
file_list = [f for f in rootdir.glob('**/*') if f.is_file()]
# For absolute paths instead of relative the current dir
file_list = [f for f in rootdir.resolve().glob('**/*') if f.is_file()]
Since Python 3.5, the glob module also supports recursive file finding:
import os
from glob import iglob
rootdir_glob = 'C:/Users/sid/Desktop/test/**/*' # Note the added asterisks
# This will return absolute paths
file_list = [f for f in iglob(rootdir_glob, recursive=True) if os.path.isfile(f)]
The file_list from either of the above approaches can be iterated over without the need for a nested loop:
for f in file_list:
print(f) # Replace with desired operations