PyQt4: Using QMutex vs QMutexLocker.

Here’s some code for my future reference on how to use QMutex or QMutexLocker.

Lessons Learned:
* Use QMutex to protect data, not code. Try not to lock hughe amounts of code within a function with mutex.lock(), mutex.unlock(), if for any reason you forget to release the lock you’ll be in trouble. Use the mutex directly only when you know what it is that you want to protect concurrent access from.
* When you have a complex function and you don’t want to worry about what to protect and when to release the lock (on exceptions thrown, before returns,etc), you can create an instance of QMutexLocker and it should release the mutex lock upon destruction… this takes us to the next lesson
* When using a QMutexLocker, DO NOT make the QMutexLocker an attribute of your class, otherwise, the reference will live after the method finishes and the lock won’t be released.

Here’s some code.

from PyQt4.Qt import QObject, QMutex, QApplication, QThread, QMutexLocker
import sys

class MutexTestSubject(QObject):
	'''
	Class that uses a QMutex to synchronize
        access to its add(),substract() methods.

        This works perfectly fine.
	'''
	def __init__(self):
		QObject.__init__(self)
		self.MAX_LIMIT = 100
		self.MIN_LIMIT = 0
		self.counter = 50
		self.mutex = QMutex()

	def add(self):
		self.mutex.lock()
		if self.counter < self.MAX_LIMIT:
			self.counter = self.counter + 1
		self.mutex.unlock()

	def substract(self):
		self.mutex.lock()
		if self.counter > self.MIN_LIMIT:
			self.counter = self.counter - 1
		self.mutex.unlock()

	def printStatus(self,thread):
		print "Counter:",self.counter," - Thread:",id(thread)

		if self.counter > self.MAX_LIMIT+1 or self.counter < self.MIN_LIMIT:
			print "Stopping Threads, Max Surpassed, Not Thread Safe. Last Thread:",id(thread)
			sys.exit()
	

class MutexLockerTestSubject(QObject):
	'''
	Class that attemps to synchronize thread
 	access to its add(),substract() methods with
	the QMutexLocker object.
	'''
	def __init__(self):
		QObject.__init__(self)
		self.MAX_LIMIT = 100
		self.MIN_LIMIT = 0
		self.counter = 50
		self.mutex = QMutex()

	def add(self):
		#VIP: DO NOT MAKE mutexLocker an attribute of your class.
		#other wise it won't be destroyed and the lock will never be released.
		mutexLocker = QMutexLocker(self.mutex)
		if self.counter < self.MAX_LIMIT:
			self.counter = self.counter + 1

	def substract(self):
		mutexLocker = QMutexLocker(self.mutex)
		if self.counter > self.MIN_LIMIT:
			self.counter = self.counter - 1

	def printStatus(self,thread):
		print "Counter:",self.counter," - Thread:",id(thread)

		if self.counter > self.MAX_LIMIT+1 or self.counter < self.MIN_LIMIT:
			print "Stopping Threads, Max Surpassed, Not Thread Safe. Last Thread:",id(thread)
			sys.exit()


class AdderWorker(QThread):
	def __init__(self, mutexTestObject):
		self.mutexTestObject = mutexTestObject
		QThread.__init__(self)

	def run(self):
		while(True):
			self.mutexTestObject.add()
			self.mutexTestObject.printStatus(self)

class SubstractorWorker(QThread):
	def __init__(self, mutexTestObject):
		self.mutexTestObject = mutexTestObject
		QThread.__init__(self,mutexTestObject)

	def run(self):
		while(True):
			self.mutexTestObject.substract()
			self.mutexTestObject.printStatus(self)

if __name__ == "__main__":
	USE_MUTEX_LOCKER = True #switch this to use regular mutexes vd QMutexLocker
	app = QApplication(sys.argv)
	mutexTestObject = MutexTestSubject() if not USE_MUTEX_LOCKER else MutexLockerTestSubject()

	adderThread  = AdderWorker(mutexTestObject)
	substracterThread = SubstractorWorker(mutexTestObject)

	adderThread.start()
	substracterThread.start()

	sys.exit(app.exec_())