le-Web.org

import os
import os.path
import time
 
 
#class used to handle one application instance mechanism
class ApplicationInstance:
 
	#specify the file used to save the application instance pid
	def __init__( self, pid_file ):
		self.pid_file = pid_file
		self.check()
		self.startApplication()
 
	#check if the current application is already running
	def check( self ):
		#check if the pidfile exists
		if not os.path.isfile( self.pid_file ):
			return
 
		#read the pid from the file
		pid = 0
		try:
			file = open( self.pid_file, 'rt' )
			data = file.read()
			file.close()
			pid = int( data )
		except:
			pass
 
		#check if the process with specified by pid exists
		if 0 == pid:
			return
 
		try:
			os.kill( pid, 0 )	#this will raise an exception if the pid is not valid
		except:
			return
 
		#exit the application
		print "The application is already running !"
		exit(0) #exit raise an exception so don't put it in a try/except block
 
	#called when the single instance starts to save it's pid
	def startApplication( self ):
		file = open( self.pid_file, 'wt' )
		file.write( str( os.getpid() ) )
		file.close()
 
	#called when the single instance exit ( remove pid file )
	def exitApplication( self ):
		try:
			os.remove( self.pid_file )
		except:
			pass
 
 
if __name__ == '__main__':
	#create application instance
	appInstance = ApplicationInstance( '/tmp/myapp.pid' )
 
	#do something here
	print "Start MyApp"
	time.sleep(5)	#sleep 5 seconds
	print "End MyApp"
 
	#remove pid file
	appInstance.exitApplication()

from applicationinstance import *
 
#create application instance
appInstance = ApplicationInstance( '/tmp/myapplication.pid' )
 
# ... execute your application
 
#remove pid file
appInstance.exitApplication()