flock.py

from fcntl import flock, LOCK_UN, LOCK_SH, LOCK_EX, LOCK_NB

__all__ = ['Flock', 'LOCK_SH', 'LOCK_EX', 'LOCK_NB']

class Flock:
    """
flock.Flock(fd, op)

    Flock object uses fcntl.flock to lock (resp. unlock)
    file descriptor (fd) with operation (op)
    when entering (resp. leaving) runtime context related to it.

    File objects providing a fileno() method are accepted as well.

    Operation is one of the following values:
        
        LOCK_SH - acquire a shared lock
        LOCK_EX - acquire an exclusive lock

    Operation can also be bitwise ORed with LOCK_NB
    to avoid blocking on lock acquisition.


Example:

    with open('/tmp/file.lock', 'w') as f:
        blocking_lock   = flock.Flock(f, flock.LOCK_EX)
        noblocking_lock = flock.Flock(f, flock.LOCK_EX|flock.LOCK_NB)

        with blocking_lock:
            pass # do something here

        try:
            with noblocking_lock:
                pass # do something else here
        except BlockingIOError:
            pass

"""

    def __init__(self, fd, op):
        (self.fd, self.op) = (fd, op)

    def __enter__(self):
        flock(self.fd, self.op)
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        flock(self.fd, LOCK_UN)