File locking in python

filelock#

This package contains a single module, which implements a platform independent file lock in Python, which provides a simple way of inter-process communication:

from filelock import Timeout, FileLock lock = FileLock("high_ground.txt.lock") with lock: with open("high_ground.txt", "a") as f: f.write("You were the chosen one.") 

Don’t use a FileLock to lock the file you want to write to, instead create a separate .lock file as shown above.

Similar libraries#

• the pid 3rd party library,
• for Windows the msvcrt module in the standard library,
• for UNIX the fcntl module in the standard library,
• the flufl.lock 3rd party library.

Installation#

filelock is available via PyPI, so you can pip install it:

python -m pip install filelock

Tutorial#

A FileLock is used to indicate another process of your application that a resource or working directory is currently used. To do so, create a FileLock first:

from filelock import Timeout, FileLock file_path = "high_ground.txt" lock_path = "high_ground.txt.lock" lock = FileLock(lock_path, timeout=1) 

The lock object supports multiple ways for acquiring the lock, including the ones used to acquire standard Python thread locks:

with lock: with open(file_path, "a") as f: f.write("Hello there!") lock.acquire() try: with open(file_path, "a") as f: f.write("General Kenobi!") finally: lock.release() @lock def decorated(): print("You're a decorated Jedi!") decorated() 

The acquire method accepts also a timeout parameter. If the lock cannot be acquired within timeout seconds, a Timeout exception is raised:

try: with lock.acquire(timeout=10): with open(file_path, "a") as f: f.write("I have a bad feeling about this.") except Timeout: print("Another instance of this application currently holds the lock.") 

The lock objects are recursive locks, which means that once acquired, they will not block on successive lock requests:

def cite1(): with lock: with open(file_path, "a") as f: f.write("I hate it when he does that.") def cite2(): with lock: with open(file_path, "a") as f: f.write("You don't want to sell me death sticks.") # The lock is acquired here. with lock: cite1() cite2() # And released here. 

Logging#

All log messages by this library are made using the DEBUG_ level , under the filelock name. On how to control displaying/hiding that please consult the logging documentation of the standard library. E.g. to hide these messages you can use:

logging.getLogger("filelock").setLevel(logging.INFO) 

FileLock vs SoftFileLock#

The FileLock is platform dependent while the SoftFileLock is not. Use the FileLock if all instances of your application are running on the same platform and a SoftFileLock otherwise.

The SoftFileLock only watches the existence of the lock file. This makes it ultra portable, but also more prone to dead locks if the application crashes. You can simply delete the lock file in such cases.

Asyncio support#

This library currently does not support asyncio. We’d recommend adding an asyncio variant though if someone can make a pull request for it, see here.

FileLocks and threads#

By default the FileLock internally uses threading.local to ensure that the lock is thread-local. If you have a use case where you’d like an instance of FileLock to be shared across threads, you can set the thread_local parameter to False when creating a lock. For example:

lock = FileLock("test.lock", thread_local=False) # lock will be re-entrant across threads # The same behavior would also work with other instances of BaseFileLock like SoftFileLock: soft_lock = SoftFileLock("soft_test.lock", thread_local=False) # soft_lock will be re-entrant across threads. 

Behavior where FileLock is thread-local started in version 3.11.0. Previous versions, were not thread-local by default.

Note: If disabling thread-local, be sure to remember that locks are re-entrant: You will be able to acquire the same lock multiple times across multiple threads.

Contributions and issues#

Contributions are always welcome, please make sure they pass all tests before creating a pull request. This module is hosted on GitHub. If you have any questions or suggestions, don’t hesitate to open a new issue 😊. There’s no bad question, just a missed opportunity to learn more.

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Importance of Filelock and how to use that in Python

We need the filelock module in Python to prevent race conditions in concurrent and multi-process applications. Race conditions occur when multiple processes or threads access the same resource simultaneously and the outcome of the application depends on which process finishes first. For example, consider a scenario where multiple processes write to the same file. If two processes try to write to the file simultaneously, the second process may overwrite the changes made by the first process, leading to data corruption and incorrect results.

To install filelock

Simple Example of filelock module to lock a file in Python:

import os from filelock import Filelock lock = FileLock("file.lock") with lock: with open("file.txt", "a") as f: f.write(f"Line written by process  os.getpid()>\n") 

Here, You can name your lockfile anything you want, in our case, it is file.lock. This file will temporally create while acquiring the lock and it will automatically deleted after releasing the lock.
we use the FileLock class from the filelock module to lock the file file.lock. The with statement is used to obtain the lock and automatically release it after the protected code has executed. This ensures that only one process can access the file at a time and prevents race conditions. The os.getpid() function is used to obtain the process ID, which is written to the file along with the line of text. This allows us to see which process wrote each line to the file.

Summary

The filelock module provides a simple and reliable way to lock files and prevent race conditions. By using a lock file, you can ensure that only one process can access a shared resource at a time, and prevent data corruption and other issues that can occur when multiple processes access the same resource simultaneously. The filelock module is a great tool for developing concurrent and multi-process applications in Python, and it provides a simple and reliable way to lock files and prevent race conditions. “Recently I had to work with filelock but I take some help from AI to understand and Implement this in my project.”

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How to Lock a File with Python FileLock

In this tutorial we will discuss how to use the Python FileLock module to Lock a File for access. This module has full support for Unix and Windows, and partial support for other platforms.

The FileLock module is used to prevent possible synchronization problems when multiple threads or processes are reading/writing to/from the same file. This type of synchronization problem is known as a Race Condition.

With FileLock we “lock” the file for the duration that a process accesses it. While this process is reading or writing to/from it, no other process will be allowed access to the file. Only when the current process is done, will another process be allowed to access it (one at a time of course). This type of condition is known as mutual exclusion.

The useful thing about this package is that we can use it to maintain synchronization between different python programs (different processes). Otherwise if you just want to maintain synchronization/mutual exclusion between threads in the same process (same python program), regular thread locks and/or binary semaphores will be enough.

How to use FileLock in Python

Using FileLock is fairly straightforward and very similar to the locking mechanisms from other Python libraries.

file = "example.txt" lockfile = "example.txt.lock"

First we will save the filepath/filename of our file in two variables as shown above. file holds the file path to the file we wish to access. lockfile holds the filepath to the lock file.

The lock file determines whether a thread/process should be allowed to access the file. (We can’t use a regular variable lock here like we usually do because we need the lock to be accessible to other processes if necessary)

The name of the lock file is typically the name of the original file + “.lock”.

lock = FileLock(lockfile, timeout = 5)

Here we create the lock using the lockfile and FileLock Class. We can also choose to pass an optional argument “ timeout ” which sets a timer on how long a process/thread will wait for the lock.

In our case, a process will wait no longer than 5 seconds for the Lock, after which it will raise a Timeout exception.

lock.acquire() try: with open(file, "a") as f: f.write("Add some data") finally: lock.release()

First we acquire the lock. We can also pass a timeout parameter here which will over-ride the timeout set earlier by FileLock() . If the lock is successfully acquired, we will go ahead and write some data to the file.

And “finally” after writing the data we will release the lock. (Using the try / except block here is not necessary)

Complete Code

Here is the complete code. Try running it yourself to see the magic!

from filelock import FileLock, Timeout file = "example.txt" lockfile = "example.txt.lock" lock = FileLock(lockfile, timeout = 5) lock.acquire() try: with open(file, "a") as f: f.write("Add some data") finally: lock.release()

Recursive File Lock

File locks are also recursive, which means that within the critical section for a lock, we may attempt to acquire it again. In certain libraries this behavior is not support, but FileLock does.

The below code features two functions, func_1 and func_2 . This first function, func_1 has the same code from the previous section with one addition. We make a call to func_2 in it, which also attempts to access the same lock.

Non-recursive locks would not allow you to access the lock again. FileLock however creates recursive locks, which can be acquired multiple times within the same critical section.

from filelock import FileLock, Timeout from threading import Lock file = "example.txt" lockfile = "example.txt.lock" lock = FileLock(lockfile, timeout = 5) def func_2(lock): lock.acquire() try: with open(file, "a") as f: f.write("Function#2: Add some data/n") finally: lock.release() def func_1(lock): lock.acquire() try: with open(file, "a") as f: f.write("Function#1: Add some data/n") func_2(lock) finally: lock.release() func_1(lock)

The code will append two new lines to an existing file or will create a new file with these two lines.

This marks the end of the How to Lock a File with Python FileLock Tutorial. Any suggestions or contributions for CodersLegacy are more than welcome. Questions regarding the tutorial content can be asked in the comments section below.

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