a Bar 对象，并调用 show 方法：

You need to instantiate a Bar object, and call its show method:

def main():
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    bar = Bar()
    bar.show()
    sys.exit(app.exec_())

如果你想在一个窗口中的按钮和状态栏，将所有的小部件放在 QMainWindow ：

import sys
from PyQt4 import QtGui, QtCore

class Bar(QtGui.QMainWindow):

    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.initUI()

    def initUI(self):
        self.setGeometry(300,300,200,200)
        self.b1 = QtGui.QPushButton("Button", self)
        self.b1.move(100,100)
        self.setWindowTitle("Example")
        self.statusBar().showMessage("Statusbar")

def main():
    app = QtGui.QApplication(sys.argv)
    bar = Bar()
    bar.show()
    sys.exit(app.exec_())

if __name__ == "__main__":
    main()