import fcntl
from contextlib import contextmanager

@contextmanager
def flocked(fd):
    """ Locks FD before entering the context, always releasing the lock. """
    try:
        fcntl.flock(fd, fcntl.LOCK_EX)
        yield
    finally:
        fcntl.flock(fd, fcntl.LOCK_UN)

if __name__ == "__main__":
    with open('test') as f:
        with flocked(f):
            print f.read()