On occasion, one requires the need to lock a file. Now, this is relatively easy if you’re targeting a specific platform because there is often a function in the library to do it for you. But what if you want to target a larger set of platforms? The following is a solution I wrote up today. It’s lockfile creation is an atomic operation and thus doesn’t suffer from any race conditions. It should work in both Windows and Unix environments.
import os import time import errno class FileLockException(Exception): pass class FileLock(object): """ A file locking mechanism that has context-manager support so you can use it in a with statement. This should be relatively cross compatible as it doesn't rely on msvcrt or fcntl for the locking. """ def __init__(self, file_name, timeout=10, delay=.05): """ Prepare the file locker. Specify the file to lock and optionally the maximum timeout and the delay between each attempt to lock. """ self.is_locked = False self.lockfile = os.path.join(os.getcwd(), "%s.lock" % file_name) self.file_name = file_name self.timeout = timeout self.delay = delay def acquire(self): """ Acquire the lock, if possible. If the lock is in use, it check again every `wait` seconds. It does this until it either gets the lock or exceeds `timeout` number of seconds, in which case it throws an exception. """ start_time = time.time() while True: try: self.fd = os.open(self.lockfile, os.O_CREAT|os.O_EXCL|os.O_RDWR) break; except OSError as e: if e.errno != errno.EEXIST: raise if (time.time() - start_time) >= self.timeout: raise FileLockException("Timeout occured.") time.sleep(self.delay) self.is_locked = True def release(self): """ Get rid of the lock by deleting the lockfile. When working in a `with` statement, this gets automatically called at the end. """ if self.is_locked: os.close(self.fd) os.unlink(self.lockfile) self.is_locked = False def __enter__(self): """ Activated when used in the with statement. Should automatically acquire a lock to be used in the with block. """ if not self.is_locked: self.acquire() return self def __exit__(self, type, value, traceback): """ Activated at the end of the with statement. It automatically releases the lock if it isn't locked. """ if self.is_locked: self.release() def __del__(self): """ Make sure that the FileLock instance doesn't leave a lockfile lying around. """ self.release()
The above class is best used in a context manager fashion through the
with statement like in the example below:
with FileLock("test.txt", timeout=2) as lock: print("Lock acquired.") # Do something with the locked file
The largest downside of this is that the directory the file is located in must be writable. I hope this code helps you. Of course, if you have a better recipe, please share it in the comments. ;)