How to change folder names in python?

You can write it out fairly straight-forward, using os.listdir and the os.path functions:

import os
basedir="C:/Test"
for fn in os.listdir(basedir):
  if not os.path.isdir(os.path.join(basedir, fn)):
    continue # Not a directory
  if ',' in fn:
    continue # Already in the correct form
  if ' ' not in fn:
    continue # Invalid format
  firstname,_,surname = fn.rpartition(' ')
  os.rename(os.path.join(basedir, fn),
            os.path.join(basedir, surname + ', ' + firstname))