grouping radio buttons in PyQt

import sys
from PyQt4 import QtCore, QtGui

class Class1(QtGui.QMainWindow):
    def __init__(self):
        super(Class1, self).__init__()
        self.func()

    def func(self):

        r0=QtGui.QRadioButton("0",self)
        r1=QtGui.QRadioButton("1",self)
        ra=QtGui.QRadioButton("a",self)
        rb=QtGui.QRadioButton("b",self)
        r0.move(100,100)
        r1.move(400,100)
        ra.move(100,400)
        rb.move(400,400)
        number_layout=QtGui.QButtonGroup()
        letter_layout=QtGui.QButtonGroup()
        number_layout.addButton(r0)
        number_layout.addButton(r1)
        letter_layout.addButton(ra)
        letter_layout.addButton(rb)
        layout=QtGui.QHBoxLayout(self)
        self.show()




def main():
    app = QtGui.QApplication(sys.argv)
    mw = Class1()
    mw.show()
    sys.exit(app.exec_())


if __name__=='__main__':
    main()

I am trying to group r0,r1 and ra,rb i.e. when r0 is checked, r1 should be unchecked with no effect on states of ra or rb. How can I achieve this? The code indicates what I have tried so far.

A QMainWindow provides a layout already, you can't simply replace that with your own. Either inherit from a plain QWidget, or create a new widget and add the layout and buttons to that.

Your naming is confusing too, QButtonGroup isn't a layout. It doesn't actually provide any visible UI. If you need a UI element that groups buttons, you should look at QGroupBox instead.

Here's a simple variation on what you have above:

def func(self):
    layout=QtGui.QHBoxLayout()  # layout for the central widget
    widget=QtGui.QWidget(self)  # central widget
    widget.setLayout(layout)

    number_group=QtGui.QButtonGroup(widget) # Number group
    r0=QtGui.QRadioButton("0")
    number_group.addButton(r0)
    r1=QtGui.QRadioButton("1")
    number_group.addButton(r1)
    layout.addWidget(r0)
    layout.addWidget(r1)

    letter_group=QtGui.QButtonGroup(widget) # Letter group
    ra=QtGui.QRadioButton("a")
    letter_group.addButton(ra)
    rb=QtGui.QRadioButton("b")
    letter_group.addButton(rb)
    layout.addWidget(ra)
    layout.addWidget(rb)

    # assign the widget to the main window
    self.setCentralWidget(widget)
    self.show()