file - python fcntl does not lock as expected (2) -

i'd use lock_ex prevent other processes modify file under modification.

process a:

from fcntl import flock, lock_ex, lock_nb time import sleep f=open("tmp.txt", "w") flock(f.fileno(), lock_ex|lock_nb) f.write("xxxx") f.flush() sleep(20) f.close() 

5 seconds after starts, process b:

f=open("tmp.txt", "w") f.close() 

and "tmp.txt" emptied process b... no ioerror raised in process b. how can 1 prevent "tmp.txt" modified 2 processes using exclusive access ?

note: "innocent" process b not use flock(), fopen() create new file. what's use of exclusive lock on file if else can modify file ? of course, if b uses flock() well, raises ioerror, if not ???

by default, flock advisory locking mechanism. more details, see this question.