What is the Python way for recursively setting file permissions?

The dirs and files lists are all always relative to root - i.e., they are the basename() of the files/folders, i.e. they don't have a / in them (or \ on windows). You need to join the dirs/files to root to get their whole path if you want your code to work to infinite levels of recursion:

import os  
path = "/tmp/foo"  
for root, dirs, files in os.walk(path):  
  for momo in dirs:  
    os.chown(os.path.join(root, momo), 502, 20)
  for momo in files:
    os.chown(os.path.join(root, momo), 502, 20)

I'm suprised the shutil module doesn't have a function for this.

As correctly pointed out above, the accepted answer misses top-level files and directories. The other answers use os.walk then loop through dirnames and filenames. However, os.walk goes through dirnames anyway, so you can skip looping through dirnames and just chown the current directory (dirpath):

def recursive_chown(path, owner):
    for dirpath, dirnames, filenames in os.walk(path):
        shutil.chown(dirpath, owner)
        for filename in filenames:
            shutil.chown(os.path.join(dirpath, filename), owner)