Demo of Permission denied exception when reading from file while another thread uses werkzeug.posixemulation.rename to replace it.

rename_bug_demo.py

from werkzeug.posixemulation import rename
from threading import Thread
from time import sleep

filename = 'test_file.dat'
running = True

def create_file():
    new_file = 'test_file.tmp'
    with open(new_file, 'w') as f:
	    f.write('abc')
    rename(new_file, filename)

def replace_file():
    while running:
        create_file()
        sleep(0.02)

create_file()

thread = Thread(target=replace_file)
thread.start()

while True:
    try:
        try:
            with open(filename) as f:
                f.read()
        except Exception, ex:
            print ex
    except KeyboardInterrupt:
        running = False
        break