Context manager that acquires an exclusive lock on a file during the managed block

locked_file_contextmanager.py

import contextlib
import fcntl
import os

@contextlib.contextmanager
def locked_file(path, mode='w'):
    """Context manager which will lock (and return a descriptor to) the passed file path on entry. The lock is
       always released on exit. Will block until the lock is obtained."""
    assert os.path.exists(path)
    with open(path, mode) as fp:
        lock = fcntl.lockf(fp, fcntl.LOCK_EX)
        try:
            yield fp
        finally:
            fcntl.lockf(fp, fcntl.LOCK_UN)