Appending to the end of a file in a concurrent environment

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 100开发者_StackOverflow7,(234)555-8917,(234)555-0077,jsmith@rosettacode.org:/home/jsmith:/bin/sh",
  "jdoe:x:1002:1000:Jane Doe,Room 1004,(234)555-8914,(234)555-0044,jdoe@rosettacode.org:/home/jdoe:/bin/sh",
  "xyz:x:1003:1000:X Yz,Room 1003,(234)555-8913,(234)555-0033,xyz@rosettacode.org:/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?)

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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

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You have to lock the file in order to ensure that nobody else is writing to it at the same time. See File Locking and lockfile or posixfile for more details.

UPDATE: And you cannot write more data into the file if the disk is full. I am not sure about Python's implementation of output re-direction, but write system call can write less bytes than requested.