window.py

from PyQt5 import QtGui

class MultiWindows(QtGui.QMainWindow):
    def __init__(self, param):
        self.__windows = []

    def addwindow(self, window):
        self.__windows.append(window)

    def show():
        for current_child_window in self.__windows:
             current_child_window.exec_() # probably show will do the same trick

class PlanetApp(QtGui.QDialog):
    def __init__(self, parent, planet):
       QtGui.QDialog.__init__(self, parent)
       # do cool stuff here

class AnimalApp(QtGui.QDialog):
    def __init__(self, parent, animal):
       QtGui.QDialog.__init__(self, parent)
       # do cool stuff here

if __name__=="__main__":
    import sys # really need this here??

    app = QtGui.QApplication(sys.argv)

    jupiter = PlanetApp(None, "jupiter")
    venus = PlanetApp(None, "venus")
    windows = MultiWindows()
    windows.addWindow(jupiter)
    windows.addWindow(venus)

    windows.show()
    app.exec_()