child GUI doesn't showup when waitcondition is used in parent pyqt

Question

I am new to PyQt coding. I am trying to launch a child process (GUI) from parent. In this I am using waitcondition and mutex to understand its functionality.Here is my code:

import sys
from PyQt4 import QtGui, QtCore
waitCondition = QtCore.QWaitCondition()
mutex = QtCore.QMutex()

class Child(QtGui.QWidget):
 def __init__(self,ABC=None):
    super(Child, self).__init__()

 def startUI(self):
    self.text = QtGui.QLineEdit()
    self.text.returnPressed.connect(self.wakeup)

 def wakeup(self):
    waitCondition.wakeAll()

class Parent(QtGui.QMainWindow):
 def __init__(self, parent=None):
    super(Parent, self).__init__()
    val = 3
    abc = Child(val) 
    abc.startUI()
    mutex.lock()
    waitCondition.wait(mutex)
    mutex.unlock()
    print ("Mutex unlocked")          

def main():
    app = QtGui.QApplication(sys.argv)
    aa = Parent()
    aa.show()
    sys.exit(app.exec_())

if __name__ == "__main__":      
 main()

If I execute this code, it doesn't launch the child GUI where as if I uncomment waitCondition.wait(mutex) in the Parent class and run it the GUI shows up.

Could some one please help me identify the mistake I am doing in here?

Answer 1

PyQt works with a main thread. The main thread will refresh the GUI only when it is not busy when your code do nothing.

QWaitCondition and QMutex are used to communicate between thread, but you have only one thread (the main one) so when you call waitCondition.wait(mutex) you stop the main thread which is wainting for a "signal" to continu. At this time the main thread is busy and can't refresh the gui and show up your child gui.