I need to lock a file for writing in Python. It will be accessed from multiple Python processes at once. I have found some solutions online, but most fail for my purposes as they are often only Unix based or Windows based.
Locking a file is usually a platform-specific operation, so you may need to allow for the possibility of running on different operating systems. For example:
import os def my_lock(f): if os.name == "posix": # Unix or OS X specific locking here elif os.name == "nt": # Windows specific locking here else: print "Unknown operating system, lock unavailable"
Coordinating access to a single file at the OS level is fraught with all kinds of issues that you probably don’t want to solve.
Your best bet is have a separate process that coordinates read/write access to that file.
There is a cross-platform file locking module here: Portalocker
Although as Kevin says, writing to a file from multiple processes at once is something you want to avoid if at all possible.
If you can shoehorn your problem into a database, you could use SQLite. It supports concurrent access and handles its own locking.
Locking is platform and device specific, but generally, you have a few options:
For all these methods, you’ll have to use a spin-lock (retry-after-failure) technique for acquiring and testing the lock. This does leave a small window for mis-synchronization, but its generally small enough to not be a major issue.
If you’re looking for a solution that is cross platform, then you’re better off logging to another system via some other mechanism (the next best thing is the NFS technique above).
Note that sqlite is subject to the same constraints over NFS that normal files are, so you can’t write to an sqlite database on a network share and get synchronization for free.
I prefer lockfile — Platform-independent file locking
I found a simple and worked(!) implementation from grizzled-python.
Simple use os.open(…, O_EXCL) + os.close() didn’t work on windows.
I have been working on a situation like this where I run multiple copies of the same program from within the same directory/folder and logging errors. My approach was to write a “lock file” to the disc before opening the log file. The program checks for the presence of the “lock file” before proceeding, and waits for its turn if the “lock file” exists.
Here is the code:
def errlogger(error): while True: if not exists('errloglock'): lock = open('errloglock', 'w') if exists('errorlog'): log = open('errorlog', 'a') else: log = open('errorlog', 'w') log.write(str(datetime.utcnow())[0:-7] + ' ' + error + 'n') log.close() remove('errloglock') return else: check = stat('errloglock') if time() - check.st_ctime > 0.01: remove('errloglock') print('waiting my turn')
After thinking over some of the comments about stale locks above I edited the code to add a check for staleness of the “lock file.” Timing several thousand iterations of this function on my system gave and average of 0.002066… seconds from just before:
lock = open('errloglock', 'w')
to just after:
so I figured I will start with 5 times that amount to indicate staleness and monitor the situation for problems.
Also, as I was working with the timing, I realized that I had a bit of code that was not really necessary:
which I had immediately following the open statement, so I have removed it in this edit.
I have been looking at several solutions to do that and my choice has been
It’s powerful and relatively well documented. It’s based on fasteners.
You may find pylocker very useful. It can be used to lock a file or for locking mechanisms in general and can be accessed from multiple Python processes at once.
If you simply want to lock a file here’s how it works:
import uuid from pylocker import Locker # create a unique lock pass. This can be any string. lpass = str(uuid.uuid1()) # create locker instance. FL = Locker(filePath='myfile.txt', lockPass=lpass, mode='w') # aquire the lock with FL as r: # get the result acquired, code, fd = r # check if aquired. if fd is not None: print fd fd.write("I have succesfuly aquired the lock !") # no need to release anything or to close the file descriptor, # with statement takes care of that. let's print fd and verify that. print fd
The scenario is like that:
The user requests a file to do something. Then, if the user sends the same request again, it informs the user that the second request is not done until the first request finishes. That’s why, I use lock-mechanism to handle this issue.
Here is my working code:
from lockfile import LockFile lock = LockFile(lock_file_path) status = "" if not lock.is_locked(): lock.acquire() status = lock.path + ' is locked.' print status else: status = lock.path + " is already locked." print status return status
The other solutions cite a lot of external code bases. If you would prefer to do it yourself, here is some code for a cross-platform solution that uses the respective file locking tools on Linux / DOS systems.
try: # Posix based file locking (Linux, Ubuntu, MacOS, etc.) # Only allows locking on writable files, might cause # strange results for reading. import fcntl, os def lock_file(f): if f.writable(): fcntl.lockf(f, fcntl.LOCK_EX) def unlock_file(f): if f.writable(): fcntl.lockf(f, fcntl.LOCK_UN) except ModuleNotFoundError: # Windows file locking import msvcrt, os def file_size(f): return os.path.getsize( os.path.realpath(f.name) ) def lock_file(f): msvcrt.locking(f.fileno(), msvcrt.LK_RLCK, file_size(f)) def unlock_file(f): msvcrt.locking(f.fileno(), msvcrt.LK_UNLCK, file_size(f)) # Class for ensuring that all file operations are atomic, treat # initialization like a standard call to 'open' that happens to be atomic. # This file opener *must* be used in a "with" block. class AtomicOpen: # Open the file with arguments provided by user. Then acquire # a lock on that file object (WARNING: Advisory locking). def __init__(self, path, *args, **kwargs): # Open the file and acquire a lock on the file before operating self.file = open(path,*args, **kwargs) # Lock the opened file lock_file(self.file) # Return the opened file object (knowing a lock has been obtained). def __enter__(self, *args, **kwargs): return self.file # Unlock the file and close the file object. def __exit__(self, exc_type=None, exc_value=None, traceback=None): # Flush to make sure all buffered contents are written to file. self.file.flush() os.fsync(self.file.fileno()) # Release the lock on the file. unlock_file(self.file) self.file.close() # Handle exceptions that may have come up during execution, by # default any exceptions are raised to the user. if (exc_type != None): return False else: return True
AtomicOpen can be used in a
with block where one would normally use an
fcntl.lockon read-only files.
from filelock import FileLock lockfile = r"c:scr.txt" lock = FileLock(lockfile + ".lock") with lock: file = open(path, "w") file.write("123") file.close()
Any code within the
with lock: block is thread-safe, meaning that it will be finished before another process has access to the file.
this worked for me:
Do not occupy large files, distribute in several small ones
you create file Temp, delete file A and then rename file Temp to A.
import os import json def Server(): i = 0 while i == 0: try: with open(File_Temp, "w") as file: json.dump(DATA, file, indent=2) if os.path.exists(File_A): os.remove(File_A) os.rename(File_Temp, File_A) i = 1 except OSError as e: print ("file locked: " ,str(e)) time.sleep(1) def Clients(): i = 0 while i == 0: try: if os.path.exists(File_A): with open(File_A,"r") as file: DATA_Temp = file.read() DATA = json.loads(DATA_Temp) i = 1 except OSError as e: print (str(e)) time.sleep(1)
If you just need Mac/POSIX this should work without external packages.
import sys import stat import os filePath = "<PATH TO FILE>" if sys.platform == 'darwin': flags = os.stat(filePath).st_flags if flags & ~stat.UF_IMMUTABLE: os.chflags(filePath, flags & stat.UF_IMMUTABLE)
and if you want to unlock a file just change,
if flags & stat.UF_IMMUTABLE: os.chflags(filePath, flags & ~stat.UF_IMMUTABLE)