Python process can't create a file in a directory, keeps getting `permission denied` IOError

Question

I've a directory mnt with 777 permissions:

drwxrwxrwx 2 root    root   14336 Feb  5 15:52 mnt

I've created a bunch of users in the group root from uid to 5204 to 5250.

I've a python script that launches a bunch of processes which:

• Change the uid to one of those I created earlier
• Create a file under mnt directory

but I keep getting IOError: [Errno 13] Permission denied:. What am I missing?

Here's the python script I am using:

from multiprocessing import Process
import random
import time
import os

def test_function(name, uid):
      os.setuid(uid)
      fname = "/home/username/mnt/file-%s-%d" % (name, random.randrange(100))
      print "Will open file: %s\n" % fname
      file = open(fname, 'w+')
      file.close

processes = []

for m in range(5204, 5212):
       name = 'Thread-%s' % str(m)
       p = Process(target=test_function, args=(name, m))
       p.start()
       processes.append(p)

for p in processes:
       p.join()

Note: I run this python script with root permissions and mnt is an NFS directory mounted with Amazon EFS