file.py

class _open_grailfile:
    def __init__(self, path):
        self.path = path
        
    def __enter__(self):
        """Lock and open the Grailfile at the given path."""
        # if the Grailfile is foobar/Grailfile, store a lock at foobar/.grail/LOCK
        dotdir_path = _get_dotgrail_dir(path)
        lock_path = dotdir_path / 'LOCK'
    
        # Don't sit there waiting for the Grailfile to be unlocked
        lock = fasteners.InterProcessLock(str(lock_path))
        with fasteners.try_lock(lock) as got:
            if not got:
                raise utils.GrailError("Grailfile is locked")
    
            with path.open('r') as f:
                lines = list(f.readlines())
                return Grailfile(lines)
            
    def __exit__(self):
        # When the context manager is exiting, write out the contents of the manifest to disk.
        with path.open('w') as f:
            grailfile.write(f)