Make sure only a single instance of a program is running

I don’t know if it’s pythonic enough, but in the Java world listening on a defined port is a pretty widely used solution, as it works on all major platforms and doesn’t have any problems with crashing programs.

Another advantage of listening to a port is that you could send a command to the running instance. For example when the users starts the program a second time, you could send the running instance a command to tell it to open another window (that’s what Firefox does, for example. I don’t know if they use TCP ports or named pipes or something like that, ‘though).

This may work.

1. Attempt create a PID file to a known location. If you fail, someone has the file locked, you’re done.

2. When you finish normally, close and remove the PID file, so someone else can overwrite it.

You can wrap your program in a shell script that removes the PID file even if your program crashes.

You can, also, use the PID file to kill the program if it hangs.

Using a lock-file is a quite common approach on unix. If it crashes, you have to clean up manually. You could stor the PID in the file, and on startup check if there is a process with this PID, overriding the lock-file if not. (However, you also need a lock around the read-file-check-pid-rewrite-file). You will find what you need for getting and checking pid in the os-package. The common way of checking if there exists a process with a given pid, is to send it a non-fatal signal.

Other alternatives could be combining this with flock or posix semaphores.

Opening a network socket, as saua proposed, would probably be the easiest and most portable.

Use a pid file. You have some known location, “/path/to/pidfile” and at startup you do something like this (partially pseudocode because I’m pre-coffee and don’t want to work all that hard):

import os, os.path
pidfilePath = """/path/to/pidfile"""
if os.path.exists(pidfilePath):
   pidfile = open(pidfilePath,"r")
   pidString = pidfile.read()
   if <pidString is equal to os.getpid()>:
      # something is real weird
      Sys.exit(BADCODE)
   else:
      <use ps or pidof to see if the process with pid pidString is still running>
      if  <process with pid == 'pidString' is still running>:
          Sys.exit(ALREADAYRUNNING)
      else:
          # the previous server must have crashed
          <log server had crashed>
          <reopen pidfilePath for writing>
          pidfile.write(os.getpid())
else:
    <open pidfilePath for writing>
    pidfile.write(os.getpid())

So, in other words, you’re checking if a pidfile exists; if not, write your pid to that file. If the pidfile does exist, then check to see if the pid is the pid of a running process; if so, then you’ve got another live process running, so just shut down. If not, then the previous process crashed, so log it, and then write your own pid to the file in place of the old one. Then continue.

Simple, cross-platform solution, found in another question by zgoda:

import fcntl
import os
import sys

def instance_already_running(label="default"):
    """
    Detect if an an instance with the label is already running, globally
    at the operating system level.

    Using `os.open` ensures that the file pointer won't be closed
    by Python's garbage collector after the function's scope is exited.

    The lock will be released when the program exits, or could be
    released if the file pointer were closed.
    """

    lock_file_pointer = os.open(f"/tmp/instance_{label}.lock", os.O_WRONLY)

    try:
        fcntl.lockf(lock_file_pointer, fcntl.LOCK_EX | fcntl.LOCK_NB)
        already_running = False
    except IOError:
        already_running = True

    return already_running

A lot like S.Lott’s suggestion, but with the code.

This code is Linux specific. It uses ‘abstract’ UNIX domain sockets, but it is simple and won’t leave stale lock files around. I prefer it to the solution above because it doesn’t require a specially reserved TCP port.

try:
    import socket
    s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    ## Create an abstract socket, by prefixing it with null. 
    s.bind( 'postconnect_gateway_notify_lock') 
except socket.error as e:
    error_code = e.args[0]
    error_string = e.args[1]
    print "Process already running (%d:%s ). Exiting" % ( error_code, error_string) 
    sys.exit (0) 

The unique string postconnect_gateway_notify_lock can be changed to allow multiple programs that need a single instance enforced.

I’m posting this as an answer because I’m a new user and Stack Overflow won’t let me vote yet.

Sorin Sbarnea’s solution works for me under OS X, Linux and Windows, and I am grateful for it.

However, tempfile.gettempdir() behaves one way under OS X and Windows and another under other some/many/all(?) *nixes (ignoring the fact that OS X is also Unix!). The difference is important to this code.

OS X and Windows have user-specific temp directories, so a tempfile created by one user isn’t visible to another user. By contrast, under many versions of *nix (I tested Ubuntu 9, RHEL 5, OpenSolaris 2008 and FreeBSD 8), the temp dir is /tmp for all users.

That means that when the lockfile is created on a multi-user machine, it’s created in /tmp and only the user who creates the lockfile the first time will be able to run the application.

A possible solution is to embed the current username in the name of the lock file.

It’s worth noting that the OP’s solution of grabbing a port will also misbehave on a multi-user machine.

Never written python before, but this is what I’ve just implemented in mycheckpoint, to prevent it being started twice or more by crond:

import os
import sys
import fcntl
fh=0
def run_once():
    global fh
    fh=open(os.path.realpath(__file__),'r')
    try:
        fcntl.flock(fh,fcntl.LOCK_EX|fcntl.LOCK_NB)
    except:
        os._exit(0)

run_once()

Found Slava-N’s suggestion after posting this in another issue (http://stackoverflow.com/questions/2959474). This one is called as a function, locks the executing scripts file (not a pid file) and maintains the lock until the script ends (normal or error).

I keep suspecting there ought to be a good POSIXy solution using process groups, without having to hit the file system, but I can’t quite nail it down. Something like:

On startup, your process sends a ‘kill -0’ to all processes in a particular group. If any such processes exist, it exits. Then it joins the group. No other processes use that group.

However, this has a race condition – multiple processes could all do this at precisely the same time and all end up joining the group and running simultaneously. By the time you’ve added some sort of mutex to make it watertight, you no longer need the process groups.

This might be acceptable if your process only gets started by cron, once every minute or every hour, but it makes me a bit nervous that it would go wrong precisely on the day when you don’t want it to.

I guess this isn’t a very good solution after all, unless someone can improve on it?

I use single_process on my gentoo;

pip install single_process

example:

from single_process import single_process

@single_process
def main():
    print 1

if __name__ == "__main__":
    main()   

refer: https://pypi.python.org/pypi/single_process/

I ran into this exact problem last week, and although I did find some good solutions, I decided to make a very simple and clean python package and uploaded it to PyPI. It differs from tendo in that it can lock any string resource name. Although you could certainly lock __file__ to achieve the same effect.

Install with: pip install quicklock

Using it is extremely simple:

[nate@Nates-MacBook-Pro-3 ~/live] python
Python 2.7.6 (default, Sep  9 2014, 15:04:36)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from quicklock import singleton
>>> # Let's create a lock so that only one instance of a script will run
...
>>> singleton('hello world')
>>>
>>> # Let's try to do that again, this should fail
...
>>> singleton('hello world')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/nate/live/gallery/env/lib/python2.7/site-packages/quicklock/quicklock.py", line 47, in singleton
    raise RuntimeError('Resource <{}> is currently locked by <Process {}: "{}">'.format(resource, other_process.pid, other_process.name()))
RuntimeError: Resource <hello world> is currently locked by <Process 24801: "python">
>>>
>>> # But if we quit this process, we release the lock automatically
...
>>> ^D
[nate@Nates-MacBook-Pro-3 ~/live] python
Python 2.7.6 (default, Sep  9 2014, 15:04:36)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from quicklock import singleton
>>> singleton('hello world')
>>>
>>> # No exception was thrown, we own 'hello world'!

Take a look: https://pypi.python.org/pypi/quicklock

linux example

This method is based on the creation of a temporary file automatically deleted after you close the application.
the program launch we verify the existence of the file;
if the file exists ( there is a pending execution) , the program is closed ; otherwise it creates the file and continues the execution of the program.

from tempfile import *
import time
import os
import sys


f = NamedTemporaryFile( prefix='lock01_', delete=True) if not [f  for f in     os.listdir('/tmp') if f.find('lock01_')!=-1] else sys.exit()

YOUR CODE COMES HERE

For anybody using wxPython for their application, you can use the function wx.SingleInstanceChecker documented here.

I personally use a subclass of wx.App which makes use of wx.SingleInstanceChecker and returns False from OnInit() if there is an existing instance of the app already executing like so:

import wx

class SingleApp(wx.App):
    """
    class that extends wx.App and only permits a single running instance.
    """

    def OnInit(self):
        """
        wx.App init function that returns False if the app is already running.
        """
        self.name = "SingleApp-%s".format(wx.GetUserId())
        self.instance = wx.SingleInstanceChecker(self.name)
        if self.instance.IsAnotherRunning():
            wx.MessageBox(
                "An instance of the application is already running", 
                "Error", 
                 wx.OK | wx.ICON_WARNING
            )
            return False
        return True

This is a simple drop-in replacement for wx.App that prohibits multiple instances. To use it simply replace wx.App with SingleApp in your code like so:

app = SingleApp(redirect=False)
frame = wx.Frame(None, wx.ID_ANY, "Hello World")
frame.Show(True)
app.MainLoop()

Here is my eventual Windows-only solution. Put the following into a module, perhaps called ‘onlyone.py’, or whatever. Include that module directly into your __ main __ python script file.

import win32event, win32api, winerror, time, sys, os
main_path = os.path.abspath(sys.modules['__main__'].__file__).replace("\", "/")

first = True
while True:
        mutex = win32event.CreateMutex(None, False, main_path + "_{<paste YOUR GUID HERE>}")
        if win32api.GetLastError() == 0:
            break
        win32api.CloseHandle(mutex)
        if first:
            print "Another instance of %s running, please wait for completion" % main_path
            first = False
        time.sleep(1)

Explanation

The code attempts to create a mutex with name derived from the full path to the script. We use forward-slashes to avoid potential confusion with the real file system.

Advantages

• No configuration or ‘magic’ identifiers needed, use it in as many different scripts as needed.
• No stale files left around, the mutex dies with you.
• Prints a helpful message when waiting

import sys,os

# start program
try:  # (1)
    os.unlink('lock')  # (2)
    fd=os.open("lock", os.O_CREAT|os.O_EXCL) # (3)  
except: 
    try: fd=os.open("lock", os.O_CREAT|os.O_EXCL) # (4) 
    except:  
        print "Another Program running !.."  # (5)
        sys.exit()  

# your program  ...
# ...

# exit program
try: os.close(fd)  # (6)
except: pass
try: os.unlink('lock')  
except: pass
sys.exit()  

On a Linux system one could also ask
pgrep -a for the number of instances, the script
is found in the process list (option -a reveals the
full command line string). E.g.

import os
import sys
import subprocess

procOut = subprocess.check_output( "/bin/pgrep -u $UID -a python", shell=True, 
                                   executable="/bin/bash", universal_newlines=True)

if procOut.count( os.path.basename(__file__)) > 1 :        
    sys.exit( ("found another instance of >{}<, quitting."
              ).format( os.path.basename(__file__)))

Remove -u $UID if the restriction should apply to all users.
Disclaimer: a) it is assumed that the script’s (base)name is unique, b) there might be race conditions.

The best solution for this on windows is to use mutexes as suggested by @zgoda.

import win32event
import win32api
from winerror import ERROR_ALREADY_EXISTS

mutex = win32event.CreateMutex(None, False, 'name')
last_error = win32api.GetLastError()

if last_error == ERROR_ALREADY_EXISTS:
   print("App instance already running")

Some answers use fctnl (included also in @sorin tendo package) which is not available on windows and should you try to freeze your python app using a package like pyinstaller which does static imports, it throws an error.

Also, using the lock file method, creates a read-only problem with database files( experienced this with sqlite3).

Building upon Roberto Rosario’s answer, I come up with the following function:

SOCKET = None
def run_single_instance(uniq_name):
    try:
        import socket
        global SOCKET
        SOCKET = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        ## Create an abstract socket, by prefixing it with null.
        # this relies on a feature only in linux, when current process quits, the
        # socket will be deleted.
        SOCKET.bind('' + uniq_name)
        return True
    except socket.error as e:
        return False

We need to define global SOCKET vaiable since it will only be garbage collected when the whole process quits. If we declare a local variable in the function, it will go out of scope after the function exits, thus the socket be deleted.

All the credit should go to Roberto Rosario, since I only clarify and elaborate upon his code. And this code will work only on Linux, as the following quoted text from https://troydhanson.github.io/network/Unix_domain_sockets.html explains:

Linux has a special feature: if the pathname for a UNIX domain socket
begins with a null byte , its name is not mapped into the
filesystem. Thus it won’t collide with other names in the filesystem.
Also, when a server closes its UNIX domain listening socket in the
abstract namespace, its file is deleted; with regular UNIX domain
sockets, the file persists after the server closes it.

Late answer, but for windows you can use:

from win32event import CreateMutex
from win32api import CloseHandle, GetLastError
from winerror import ERROR_ALREADY_EXISTS
import sys

class singleinstance:
    """ Limits application to single instance """

    def __init__(self):
        self.mutexname = "testmutex_{D0E858DF-985E-4907-B7FB-8D732C3FC3B9}"
        self.mutex = CreateMutex(None, False, self.mutexname)
        self.lasterror = GetLastError()
    
    def alreadyrunning(self):
        return (self.lasterror == ERROR_ALREADY_EXISTS)
        
    def __del__(self):
        if self.mutex:
            CloseHandle(self.mutex)

Usage

# do this at beginnig of your application
myapp = singleinstance()

# check is another instance of same program running
if myapp.alreadyrunning():
    print ("Another instance of this program is already running")
    sys.exit(1)

Here is a cross platform example that I’ve tested on Windows Server 2016 and Ubuntu 20.04 using Python 3.7.9:

import os

class SingleInstanceChecker:
    def __init__(self, id):
        if isWin():
            ensure_win32api()
            self.mutexname = id
            self.lock = win32event.CreateMutex(None, False, self.mutexname)
            self.running = (win32api.GetLastError() == winerror.ERROR_ALREADY_EXISTS)

        else:
            ensure_fcntl()
            self.lock = open(f"/tmp/isnstance_{id}.lock", 'wb')
            try:
                fcntl.lockf(self.lock, fcntl.LOCK_EX | fcntl.LOCK_NB)
                self.running = False
            except IOError:
                self.running = True


    def already_running(self):
        return self.running
        
    def __del__(self):
        if self.lock:
            try:
                if isWin():
                    win32api.CloseHandle(self.lock)
                else:
                    os.close(self.lock)
            except Exception as ex:
                pass

# ---------------------------------------
# Utility Functions
# Dynamically load win32api on demand
# Install with: pip install pywin32
win32api=winerror=win32event=None
def ensure_win32api():
    global win32api,winerror,win32event
    if win32api is None:
        import win32api
        import winerror
        import win32event


# Dynamically load fcntl on demand
# Install with: pip install fcntl
fcntl=None
def ensure_fcntl():
    global fcntl
    if fcntl is None:
        import fcntl


def isWin():
    return (os.name == 'nt')
# ---------------------------------------

Here is it in use:

import time, sys

def main(argv):
    _timeout = 10
    print("main() called. sleeping for %s seconds" % _timeout)
    time.sleep(_timeout)
    print("DONE")


if __name__ == '__main__':
    SCR_NAME = "my_script"
    sic = SingleInstanceChecker(SCR_NAME)
    if sic.already_running():
        print("An instance of {} is already running.".format(SCR_NAME))
        sys.exit(1)
    else:
        main(sys.argv[1:])

Here’s a good example for django with contextmanager and memcached:
https://docs.celeryproject.org/en/latest/tutorials/task-cookbook.html

Can be used to protect simultaneous operation on different hosts.
Can be used to manage multiple tasks.
Can also be changed for simple python scripts.

My modification of the above code is here:

import time
from contextlib import contextmanager
from django.core.cache import cache


@contextmanager
def memcache_lock(lock_key, lock_value, lock_expire):
    timeout_at = time.monotonic() + lock_expire - 3

    # cache.add fails if the key already exists
    status = cache.add(lock_key, lock_value, lock_expire)
    try:
        yield status
    finally:
        # memcache delete is very slow, but we have to use it to take
        # advantage of using add() for atomic locking
        if time.monotonic() < timeout_at and status:
            # don't release the lock if we exceeded the timeout
            # to lessen the chance of releasing an expired lock owned by someone else
            # also don't release the lock if we didn't acquire it
            cache.delete(lock_key)


LOCK_EXPIRE = 60 * 10  # Lock expires in 10 minutes


def main():
    lock_name, lock_value = "lock_1", "locked"
    with memcache_lock(lock_name, lock_value, LOCK_EXPIRE) as acquired:
        if acquired:
            # single instance code here:
            pass


if __name__ == "__main__":
    main()

Here is a cross-platform implementation, creating a temporary lock file using a context manager.

Can be used to manage multiple tasks.

import os
from contextlib import contextmanager
from time import sleep


class ExceptionTaskInProgress(Exception):
    pass


# Context manager for suppressing exceptions
class SuppressException:
    def __init__(self):
        pass

    def __enter__(self):
        return self

    def __exit__(self, *exc):
        return True


# Context manager for task
class TaskSingleInstance:
    def __init__(self, task_name, lock_path):
        self.task_name = task_name
        self.lock_path = lock_path
        self.lock_filename = os.path.join(self.lock_path, self.task_name + ".lock")

        if os.path.exists(self.lock_filename):
            raise ExceptionTaskInProgress("Resource already in use")

    def __enter__(self):
        self.fl = open(self.lock_filename, "w")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.fl.close()
        os.unlink(self.lock_filename)


# Here the task is silently interrupted
# if it is already running on another instance.
def main1():
    task_name = "task1"
    tmp_filename_path = "."
    with SuppressException():
        with TaskSingleInstance(task_name, tmp_filename_path):
            print("The task `{}` has started.".format(task_name))
            # The single task instance code is here.
            sleep(5)
            print("The task `{}` has completed.".format(task_name))


# Here the task is interrupted with a message
# if it is already running in another instance.
def main2():
    task_name = "task1"
    tmp_filename_path = "."
    try:
        with TaskSingleInstance(task_name, tmp_filename_path):
            print("The task `{}` has started.".format(task_name))
            # The single task instance code is here.
            sleep(5)
            print("Task `{}` completed.".format(task_name))
    except ExceptionTaskInProgress as ex:
        print("The task `{}` is already running.".format(task_name))


if __name__ == "__main__":
    main1()
    main2()