QMutexLocker

Synopsis

Functions

• def __enter__ ()
• def __exit__ (arg__1, arg__2, arg__3)
• def mutex ()
• def relock ()
• def unlock ()

Detailed Description

The PySide.QtCore.QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes.

Locking and unlocking a PySide.QtCore.QMutex in complex functions and statements or in exception handling code is error-prone and difficult to debug. PySide.QtCore.QMutexLocker can be used in such situations to ensure that the state of the mutex is always well-defined.

PySide.QtCore.QMutexLocker should be created within a function where a PySide.QtCore.QMutex needs to be locked. The mutex is locked when PySide.QtCore.QMutexLocker is created. You can unlock and relock the mutex with unlock() and relock() . If locked, the mutex will be unlocked when the PySide.QtCore.QMutexLocker is destroyed.

For example, this complex function locks a PySide.QtCore.QMutex upon entering the function and unlocks the mutex at all the exit points:

def complexFunction(flag):
    mutex.lock()

    retVal = 0

    if flag == 0 or flag == 1:
        mutex.unlock()
        return moreComplexFunction(flag)
    elif flag == 2:
        status = anotherFunction()
        if status < 0:
            mutex.unlock()
            return -2
        retVal = status + flag
    else:
        if flag > 10:
            mutex.unlock()
            return -1

    mutex.unlock()
    return retVal

This example function will get more complicated as it is developed, which increases the likelihood that errors will occur.

Using PySide.QtCore.QMutexLocker greatly simplifies the code, and makes it more readable:

def complexFunction(flag):
    locker = QMutexLocker(mutex)

    retVal = 0

    if flag == 0 or flag == 1:
        return moreComplexFunction(flag)
    elif flag == 2:
            status = anotherFunction()
            if status < 0:
                return -2
            retVal = status + flag
    else:
        if flag > 10:
            return -1

    return retVal

Now, the mutex will always be unlocked when the PySide.QtCore.QMutexLocker object is destroyed (when the function returns since locker is an auto variable).

The same principle applies to code that throws and catches exceptions. An exception that is not caught in the function that has locked the mutex has no way of unlocking the mutex before the exception is passed up the stack to the calling function.

PySide.QtCore.QMutexLocker also provides a mutex() member function that returns the mutex on which the PySide.QtCore.QMutexLocker is operating. This is useful for code that needs access to the mutex, such as QWaitCondition.wait() . For example:

class SignalWaiter:
    def __init__(mutex):
        self.locker = mutex

    def waitForSignal():
        # ...
        while not signalled:
            waitCondition.wait(self.locker.mutex())
        # ...

See also

PySide.QtCore.QReadLocker PySide.QtCore.QWriteLocker PySide.QtCore.QMutex

class PySide.QtCore.QMutexLocker(m)

Parameters:m – PySide.QtCore.QMutex

Constructs a PySide.QtCore.QMutexLocker and locks mutex . The mutex will be unlocked when the PySide.QtCore.QMutexLocker is destroyed. If mutex is zero, PySide.QtCore.QMutexLocker does nothing.

See also

QMutex.lock()

PySide.QtCore.QMutexLocker.__enter__()

PySide.QtCore.QMutexLocker.__exit__(arg__1, arg__2, arg__3)

Parameters:

• arg__1 – PyObject
• arg__2 – PyObject
• arg__3 – PyObject

PySide.QtCore.QMutexLocker.mutex()

Return type:PySide.QtCore.QMutex

Returns a pointer to the mutex that was locked in the constructor.

PySide.QtCore.QMutexLocker.relock()

Relocks an unlocked mutex locker.

See also

PySide.QtCore.QMutexLocker.unlock()

PySide.QtCore.QMutexLocker.unlock()

Unlocks this mutex locker. You can use relock() to lock it again. It does not need to be locked when destroyed.

See also

PySide.QtCore.QMutexLocker.relock()