from PyQt4.QtGui import *
from PyQt4.QtCore import *
from time import sleep
 
class Widget(QWidget):
    def __init__(self):
        QWidget.__init__(self)
        self.__lab = QLabel("Text", self)
        QTimer.singleShot(2000, self.changeFont)
        edit = QLineEdit(self)

        lay = QGridLayout(self)
        lay.addWidget(self.__lab, 0, 0)
        lay.addWidget(edit, 0, 1)

    def changeFont(self):
        f = self.__lab.font()
        f.setBold(True);
        self.__lab.setFont(f);
        # self.__lab.update()
        print "Changed font"
 
app = QApplication([])
w = Widget()
w.show()
app.exec_()