class window_b(QtGui.QDialog):
    def __init__(self,parent=None):
        super(window_b, self).__init__(parent)
        window_a.setEnabled(False)
        self.ui = Ui_Form_window_b()
        self.ui.setupUi(self)
        self.setFocusPolicy(QtCore.Qt.StrongFocus)
    def mouseMoveEvent (self,event):
        source= self.sender()
        #print source.name()
        # The action I want to do when the mouse is over the button:
        source.setStyleSheet("background-color:#66c0ff;border-radiu‌​s: 5px;")

print source.name()
AttributeError: 'NoneType' object has no attribute 'name'

class window_b(QtGui.QDialog):
    def __init__(self,parent=None):
        super(window_b, self).__init__(parent)
        window_a.setEnabled(False)
        self.ui = Ui_Form_window_b()
        self.ui.setupUi(self)
        self.setFocusPolicy(QtCore.Qt.StrongFocus)

        # Get all the buttons (you probably don't want all of them)
        buttons = self.findChildren(QtGui.QAbstractButton)
        for button in buttons:
            button.installEventFilter(self)

    def eventFilter(self, obj, event):
        if event.type() == QtCore.QEvent.Enter:
            print("mouse entered %s" % obj.objectName())
        elif event.type() == QtCore.QEvent.Leave:
            print("mouse leaved %s" % obj.objectName())
        return super(window_b, self).eventFilter(obj, event)

QPushButton {
     border: 1px solid black;
     padding: 5px;
}
QPushButton:hover {
    border: 1px solid black;
    border-radius: 5px;
    background-color:#66c0ff;
}