What steps need to be taken to ensure that "full" lines are always correctly appended to the end of a file if multiple of the following (example) program are running concurrently.

#!/usr/bin/env python
import random
passwd_text=open("passwd.txt","a+")
u=("jsmith:x:1001:1000:Joe Smith,Room 1007,(234)555-8917,(234)555-0077,[email protected]:/home/jsmith:/bin/sh","jdoe:x:1002:1000:Jane Doe,Room 1004,(234)555-8914,(234)555-0044,[email protected]:/home/jdoe:/bin/sh","xyz:x:1003:1000:X Yz,Room 1003,(234)555-8913,(234)555-0033,[email protected]:/home/xyz:/bin/sh")
for i in range(random.randint(1,2)):print >> passwd_text, random.choice(u)
passwd_text.close()

And: Can an "all or nothing" append be guaranteed (on linux/unix) even if the the disk becomes full, or "ulimit -f" has been set?

(Note similar question: How do you append to a file?)

I think the discussion of this "bug" in python's normal open function suggests that you don't get the POSIX atomic guarantee, but you do if you use

with io.open('myfile', 'a') as f:f.write('stuff')

http://docs.python.org/2/library/io.html#io.open

if the operating system implements its write sys call correctly...

http://bugs.python.org/issue15723