Python shelve module question

Question

Does the Python shelve module have any protection built in to make sure two processes aren\'t writing to a file at the same time?

Answer 1

I've implemented Ivo's approach as a context manager, for anyone interested:

from contextlib import contextmanager, closing
from fcntl import flock, LOCK_SH, LOCK_EX, LOCK_UN
import shelve

@contextmanager
def locking(lock_path, lock_mode):
    with open(lock_path, 'w') as lock:
        flock(lock.fileno(), lock_mode) # block until lock is acquired
        try:
            yield
        finally:
            flock(lock.fileno(), LOCK_UN) # release

class DBManager(object):
    def __init__(self, db_path):
        self.db_path = db_path

    def read(self):
        with locking("%s.lock" % self.db_path, LOCK_SH):
            with closing(shelve.open(self.db_path, "c", 2)) as db:
                return dict(db)

    def cas(self, old_db, new_db):
        with locking("%s.lock" % self.db_path, LOCK_EX):
            with closing(shelve.open(self.db_path, "c", 2)) as db:
                if old_db != dict(db):
                    return False
                db.clear()
                db.update(new_db)
                return True