Locking a file in Python

Alright, so I ended up going with the code I wrote here, on my website ^{link is dead, view on archive.org} (also available on GitHub). I can use it in the following fashion:

from filelock import FileLock

with FileLock("myfile.txt"):
    # work with the file as it is now locked
    print("Lock acquired.")

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        

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

WARNINGS:

• If running on Windows and Python crashes before exit is called, I'm not sure what the lock behavior would be.
• The locking provided here is advisory, not absolute. All potentially competing processes must use the "AtomicOpen" class.
• As of (Nov 9th, 2020) this code only locks writable files on Posix systems. At some point after the posting and before this date, it became illegal to use the fcntl.lock on read-only files.

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:

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

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.

Here's an example of how to use the filelock library, which is similar to Evan Fossmark's implementation:

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.

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"

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)

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