Two processes reading/writing to the same file Python

Question

I have one process who's reading from a file (using file.read()) and one process who's writing to the same file (file.write()). The problem is it doesn't work - I get no errors but they can't operate at the same time. I've tried making the read and write operations none-blocking and then flushing the stream, as follows:

fcntl.fcntl(file, fcntl.F_SETFL, os.O_NONBLOCK)
file.write(msg)
file.flush()

Am I completely misunderstanding it? How should one accomplish writing and reading to one file from different processes?

Answer 1

Take a look at this Read-Write Lock class:

• http://code.activestate.com/recipes/502283-read-write-lock-class-rlock-like/

and at this articles about locking and threading:

• http://effbot.org/zone/thread-synchronization.htm
• http://linuxgazette.net/107/pai.html

Answer 2

Is there a reason to use a common file? Inter-process communication is probably much easier using sockets.

Answer 3

test1.py

import os
f = open('txt.txt', 'a', os.O_NONBLOCK)
while 1:
        f.write('asd')
        f.flush()

test2.py

import os
f = open('txt.txt', 'r', os.O_NONBLOCK)
while 1:
    print f.read(3)

This works fine for me.