Cross Platform Example of Single Instance Checking in Python

SingleInstanceChecker.py

import time, sys, 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 (python stdlib)
fcntl=None
def ensure_fcntl():
    global fcntl
    if fcntl is None:
        import fcntl


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


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:])

Great job @JavaScriptDude !

Where is the fcntl package from because get these errors?:

> pip install fcntl
ERROR: Could not find a version that satisfies the requirement fcntl (from versions: none)
ERROR: No matching distribution found for fcntl

@dineshbvadhia Was a typo. fcntl is built into the python stdlib. Thanks for letting me know.

Hi. Typed the code but mypy generates a bunch of 'has no attribute' errors which I'm not sure how to resolve:

from typing import Any
import time, sys, os

class SingleInstanceChecker:
    def __init__(self, id: str) -> None:
        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) -> Any:
        return self.running
        
    def __del__(self) -> None:
        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() -> None:
    global win32api,winerror,win32event
    if win32api is None:
        import win32api             
        import winerror             
        import win32event           

# Dynamically load fcntl on demand (python stdlib)
fcntl=None
def ensure_fcntl() -> None:
    global fcntl
    if fcntl is None:
        import fcntl


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

def main(argv: list[str]) -> None:
    _timeout: int = 10
    print("main() called. sleeping for %s seconds" % _timeout)
    time.sleep(_timeout)
    print("DONE")


if __name__ == '__main__':
    SCR_NAME: str = "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:])

> mypy SingleInstanceChecker.py
py:9: error: "None" has no attribute "CreateMutex"  [attr-defined]
py:10: error: "None" has no attribute "GetLastError"  [attr-defined]
py:10: error: "None" has no attribute "ERROR_ALREADY_EXISTS"  [attr-defined]
py:16: error: "None" has no attribute "lockf"  [attr-defined]
py:16: error: "None" has no attribute "LOCK_EX"  [attr-defined]
py:16: error: "None" has no attribute "LOCK_NB"  [attr-defined]
py:28: error: "None" has no attribute "CloseHandle"  [attr-defined]
Found 7 errors in 1 file (checked 1 source file)

Although I've never used mypy. I suspect that mypy is getting confused by the dynamic loading of the platform specific library via ensure_win32api and ensure_fcntl. I'm sure if you made a linux and windows specific versions and explicitly load the respective library, mypy will not have issues.

There may be a more pythonic way to do this type of platform specific dynamic loading that would make mypy happy but I don't have the bandwidth to investigate. Suggestions would be appreciated.

The code definitely works and I've used it in dozens of usecases where I develop the code in linux and deploy in windows in some cases. I use this code for my scheduled task / cron based jobs.

@JavaScriptDude Apologies if I gave the impression the code doesn't work - it does work. My code base is typed and so wanted to type this. I'll investigate further.