List directory tree structure in python?

I know that we can use os.walk() to list all sub-directories or all files in a directory. However, I would like to list the full directory tree content:

- Subdirectory 1:
   - file11
   - file12
   - Sub-sub-directory 11:
         - file111
         - file112
- Subdirectory 2:
    - file21
    - sub-sub-directory 21
    - sub-sub-directory 22    
        - sub-sub-sub-directory 221
            - file 2211

How to best achieve this in Python?

Here's a function to do that with formatting:

import os

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