Why the mutex is not locked

I have a mutex problem, I can’t understand why the part of the code between locking and unlocking is executed simultaneously in all threads. Here is my class:

class MyThread(QtCore.QThread):
    mutex = QtCore.QMutex()
    load_message_input = QtCore.pyqtSignal()

    def __init__(self, id, window):
        super(MyThread, self).__init__()
        self.id = id
        self.window = window

    def run(self):
        print "Thread %d is started" % self.id
        self.get_captcha_value()
        print "Thread %d is finishing" % self.id

    def get_captcha_value(self):
        MyThread.mutex.lock()
        print "Thread %d locks mutex" % self.id
        self.load_message_input.connect(self.window.show_input)
        self.load_message_input.emit()
        self.window.got_message.connect(self.print_message)
        self.window.input_finished.wait(self.mutex)
        print "Thread %d unlocks mutex" % self.id
        MyThread.mutex.unlock()

    @QtCore.pyqtSlot("QString")
    def print_message(self, msg):
        print "Thread %d: %s" % (self.id, msg)

This is how I describe the window:

class MyDialog(QtGui.QDialog):
    got_message = QtCore.pyqtSignal("QString")

    def __init__(self, *args, **kwargs):
        super(MyDialog, self).__init__(*args, **kwargs)
        self.last_message = None

        self.setModal(True)
        self.message_label = QtGui.QLabel(u"Message")
        self.message_input = QtGui.QLineEdit()
        self.dialog_buttons = QtGui.QDialogButtonBox(QtGui.QDialogButtonBox.Ok | QtGui.QDialogButtonBox.Cancel)
        self.dialog_buttons.accepted.connect(self.accept)
        self.dialog_buttons.rejected.connect(self.reject)
        self.hbox = QtGui.QHBoxLayout()
        self.hbox.addWidget(self.message_label)
        self.hbox.addWidget(self.message_input)
        self.vbox = QtGui.QVBoxLayout()
        self.vbox.addLayout(self.hbox)
        self.vbox.addWidget(self.dialog_buttons)
        self.setLayout(self.vbox)

        self.input_finished = QtCore.QWaitCondition()

    @QtCore.pyqtSlot()
    def show_input(self):
        print "showing input"
        self.show()
        self.setModal(True)

    @QtCore.pyqtSlot()
    def on_accepted(self):
        print "emit: ", self.message_input.text()
        self.got_message.emit(self.message_input.text())
        self.input_finished.wakeAll()

And here is the main topic:

import sys
app = QtGui.QApplication(sys.argv)

window = test_qdialog.MyDialog()
threads = []

for i in range(5):
    thread = MyThread(i, window)
    if not thread.isRunning():
        thread.start()
        threads.append(thread)

sys.exit(app.exec_())

The result is as follows:

Thread 0 is startedThread 1 is startedThread 4 is started


Thread 0 locks mutexThread 3 is started
Thread 2 is started

Thread 2 locks mutex
Thread 3 locks mutex
Thread 1 locks mutex
Thread 4 locks mutex
showing input
showing input
showing input
showing input
showing input

UPDATE: Thanks to Joanne for his suggestions. Here's what the code for the MyThread class looks like:

class MyThread(QtCore.QThread):
    mutex = QtCore.QMutex()
    load_message_input = QtCore.pyqtSignal()

    def __init__(self, id, window):
        super(MyThread, self).__init__()
        self.id = id
        self.window = window
        # self.mutex = QtCore.QMutex()
        self.load_message_input.connect(self.window.show_input)

    def run(self):
        print "Thread %d is started" % self.id
        self.get_captcha_value()
        print "Thread %d is finishing" % self.id

    def get_captcha_value(self):
        MyThread.mutex.lock()
        print "Thread %d locks mutex" % self.id
        self.load_message_input.emit()
        mutex2 = QtCore.QMutex()
        mutex2.lock()
        self.window.got_message.connect(self.print_message)
        self.window.input_finished.wait(mutex2)
        mutex2.unlock()
        self.window.got_message.disconnect(self.print_message)
        print "Thread %d unlocks mutex" % self.id
        MyThread.mutex.unlock()

    @QtCore.pyqtSlot("QString")
    def print_message(self, msg):
        print "Thread %d: %s" % (self.id, msg)

Now I get this this exception after the completion of the first thread:

Traceback (most recent call last):
  File "/path/to/script/qdialog_threads.py", line 20, in run
    self.get_captcha_value()
  File "path/to/script/qdialog_threads.py", line 34, in get_captcha_value
    MyThread.mutex.unlock()
AttributeError: 'NoneType' object has no attribute 'mutex'