python - How to get the "result" from a contextmanager using 'with' -

i found demo of file locking class (here: https://groups.google.com/forum/#!topic/pug-pe/mqr7kx-cenu), don't quite understand mechanics of using it.

@contextmanager def filelock(lock_file):     if os.path.exists(lock_file):         print 'only 1 script can run @ once. '\               'script locked %s' % lock_file         sys.exit(-1)     else:         open(lock_file, 'w').write("1")         try:             yield         finally:             os.remove(lock_file) 

i believe says "if file passed in not exist, open it. when done, delete it.

the demonstrated use is:

with filelock('/tmp/my_script.lock'):     print "i here , sleeping 10 sec..."     time.sleep(10) 

and works correctly - if run script once see "i here , sleeping 10 sec...", , if run again within 10 seconds, see "only 1 script can run @ once. script locked /tmp/my_script.lock". however, use file lock, 1 typically want "wait until lock obtained" before doing something. however, 'sys.exit()' seems prevent that. seems want wrap 'with' in while loop somehow? like:

while fileislocked:     filelock('/tmp/my_script.lock'): # try lock         print "i here , sleeping 10 sec..."         time.sleep(10) 

but don't understand how return value filelock. can please explain how this?

you should use following:

 @contextmanager def filelock(lock_file):     while os.path.exists(lock_file):         print 'only 1 script can run @ once. '\               'script locked %s' % lock_file         time.sleep(1)     open(lock_file, 'w').write("1")     try:         yield     finally:         os.remove(lock_file) 

this directly answers declared need. otoh method buggy because there explicit race condition between checking file existence , opening one. more stable approach should use o_excl ensure file didn't exist during creation, or flock use locking on file contents, not presence. also, kernel-level ipc can used (posix semaphores, sysv semaphores, etc.)