He estado intentando llamar a una nueva ventana desde una existente utilizando python3 y Qt4.

He creado dos ventanas usando Qt Designer (la aplicación principal y otra), y he convertido los archivos .ui generados por Qt Designer en scripts .py, pero parece que no puedo crear nuevas ventanas desde la aplicación principal .

Intenté hacer esto:

############### MAIN APPLICATION SCRIPT ################

from PyQt4 import QtCore, QtGui
import v2

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    _fromUtf8 = lambda s: s

class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName(_fromUtf8("Form"))
        Form.resize(194, 101)
        self.button1 = QtGui.QPushButton(Form)
        self.button1.setGeometry(QtCore.QRect(50, 30, 99, 23))
        self.button1.setObjectName(_fromUtf8("button1"))

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        Form.setWindowTitle(QtGui.QApplication.translate("Form", "Form", None, QtGui.QApplication.UnicodeUTF8))
        self.button1.setText(QtGui.QApplication.translate("Form", "Ventana", None, QtGui.QApplication.UnicodeUTF8))

        self.button1.connect(self.button1, QtCore.SIGNAL(_fromUtf8("clicked()")), self.mbutton1)

    def mbutton1(self):
        v2.main()



if __name__ == "__main__":
    import sys
    app = QtGui.QApplication(sys.argv)
    Form = QtGui.QWidget()
    ui = Ui_Form()
    ui.setupUi(Form)
    Form.show()
    sys.exit(app.exec_())

################## SECOND WINDOW #######################

from PyQt4 import QtCore, QtGui

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    _fromUtf8 = lambda s: s

class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName(_fromUtf8("Form"))
        Form.resize(400, 300)
        self.label = QtGui.QLabel(Form)
        self.label.setGeometry(QtCore.QRect(160, 40, 57, 14))
        self.label.setObjectName(_fromUtf8("label"))

        self.retranslateUi(Form)
        QtCore.QMetaObject.connectSlotsByName(Form)

    def retranslateUi(self, Form):
        Form.setWindowTitle(QtGui.QApplication.translate("Form", "Form", None, QtGui.QApplication.UnicodeUTF8))
        self.label.setText(QtGui.QApplication.translate("Form", "LABEL 2", None, QtGui.QApplication.UnicodeUTF8))

def main():
    import sys
    app = QtGui.QApplication(sys.argv)
    Form = QtGui.QWidget()
    ui = Ui_Form()
    ui.setupUi(Form)
    Form.show()
    sys.exit(app.exec_())

Pero me sale este mensaje de error:

 QCoreApplication::exec: The event loop is already running
 QPixmap: Must construct a QApplication before a QPaintDevice