I used pyqt to build a little gui where you can choose a `Key` from a QComboBox and the `Value` is taken to do a math calculation that usually takes 3 seconds and the result is a short string. I'm updating the calculated short string in the gui in a textbox. This is my code (I removed the math calculation code because it's not relevant), it is working so far:

```
import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *
import os 
dict1 = {"Key":Value, "Key2":Value2, "Key3":Value3} # Value are int's

class combodemo(QWidget):
  def __init__(self, parent = None):
    super(combodemo, self).__init__(parent)

    layout = QHBoxLayout()
    self.cb = QComboBox()
    self.cb.addItems([key for key in sorted(dict1.keys())])
    self.cb.currentIndexChanged.connect(self.selectionchange)

    layout.addWidget(self.cb)
    self.setLayout(layout)
    self.cb.textbox = QLineEdit(self)
    self.cb.textbox.move(100, 200)
    self.cb.textbox.resize(150,50)
    self.cb.textbox.setAlignment(Qt.AlignCenter)
    self.cb.textbox.setText("Initial Text")

  def selectionchange(self):
    #self.cb.textbox.setText("Calculating...") # THIS IS NOT WORKING
    self.cb.currentIndexChanged.connect(self.selectionchange)
    # MATH CALCULATION CODE GOES HERE[...]
    self.cb.textbox.setText("RESULT OF MATH CALCULATION")

def main():
  app = QApplication(sys.argv)
  ex = combodemo()
  ex.show()
  sys.exit(app.exec_())

if __name__ == '__main__':
  main()

```

Right now the program freezes for 3 seconds after a `Key` is selected (because of the math calculation I am doing). Since my math calculation takes 3 seconds, I would like to update the textbox to `"Calculating..."` once a `Key` from the QComboBox is selected. So while my math calculation is performed I want the textbox to show `"Calculating..."`. Once the calculation is done it should simple rewrite the textbox and show the math result.

In the above code I tried to achieve it in the function `def selectionchange(self)` right at the beginning (it's uncommented). But it has no effect. How can I make this work?