Bdtjtk

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.

I prefer lockfile — Platform-independent file locking

Locking is platform and device specific, but generally, you have a few options:

1. Use flock(), or equivalent (if your os supports it). This is advisory locking, unless you check for the lock, its ignored.


2. Use a lock-copy-move-unlock methodology, where you copy the file, write the new data, then move it (move, not copy - move is an atomic operation in Linux -- check your OS), and you check for the existence of the lock file.


3. Use a directory as a "lock". This is necessary if you're writing to NFS, since NFS doesn't support flock().


4. There's also the possibility of using shared memory between the processes, but I've never tried that; it's very OS-specific.

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 an 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.

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.)

    import fcntl, os

    def lock_file(f):

        fcntl.lockf(f, fcntl.LOCK_EX)

    def unlock_file(f):

        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        

Now, AtomicOpen can be used in a with block where one would normally use an open statement.

WARNING: If running on Windows and Python crashes before exit is called, I'm not sure what the lock behavior would be.

WARNING: The locking provided here is advisory, not absolute. All potentially competing processes must use the "AtomicOpen" class.

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.

I have been looking at several solutions to do that and my choice has been
oslo.concurrency

It's powerful and relatively well documented. It's based on fasteners.

Other solutions:

• 
  Portalocker: requires pywin32, which is an exe installation, so not possible via pip


• 
  fasteners: poorly documented


• 
  lockfile: deprecated


• 
  flufl.lock: NFS-safe file locking for POSIX systems.


• 
  simpleflock : Last update 2013-07


• 
  zc.lockfile : Last update 2016-06 (as of 2017-03)


• 
  lock_file : Last update in 2007-10

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"

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

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')

EDIT---
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:

remove('errloglock')

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:

lock.close()

which I had immediately following the open statement, so I have removed it in this edit.

I found a simple and worked(!) implementation from grizzled-python.

Simple use os.open(..., O_EXCL) + os.close() didn't work on windows.

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