继承的类无法使用exec调用python中的基本方法

Question

I have following code as text file (for example: code.txt)

from PyQt4 import QtGui, QtCore

class Window(QtGui.QWidget):
    def __init__(self):
        QtGui.QWidget.__init__(self)
        self.button = QtGui.QPushButton('Test', self)
        self.button.clicked.connect(self.handleButton)
        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.button)

    def handleButton(self):
        str = '''
class A:
    def a(self):
        print('Hello')
        pass

class B(A):
    def a(self):
        super(B,self).a()
        print('World')
    pass    

b = B()
b.a()
'''    
        exec(str )        
if __name__ == '__main__':

    import sys
    app = QtGui.QApplication(sys.argv)
    window = Window()
    window.show()
    sys.exit(app.exec_())

run this command, I got following error:

Traceback (most recent call last):
  File "C:/Users/xxx/xxx/w.py", line 28, in handleButton
    exec(str )        
  File "<string>", line 14, in <module>
  File "<string>", line 9, in a
NameError: name 'B' is not defined

without 'super(B,self).a()', it is ok. And if execute code in code.txt, it is also no problem

Answer 1

Change code as following works:

from PyQt4 import QtGui, QtCore

class Window(QtGui.QWidget):
    def __init__(self):
        QtGui.QWidget.__init__(self)
        self.button = QtGui.QPushButton('Test', self)
        self.button.clicked.connect(self.handleButton)
        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.button)

    def handleButton(self):
        str = '''
class A:
    def a(self):
        print('Hello')
        pass

class B(A):
    def a(self):
        super(B,self).a()
        print('World')
    pass    

b = B()
b.a()
'''    
        exec(str,globals() )        
if __name__ == '__main__':

    import sys
    app = QtGui.QApplication(sys.argv)
    window = Window()
    window.show()
    sys.exit(app.exec_())

you can see only difference is exec(str) and exec(str,globals())