A Qapplication Instance Already Exists

I'm doing some simple PySide on 3Dsmax 2015. This is my error: python.ExecuteFile 'C:\Program Files\Autodesk\3ds Max 2015\scripts\Python\demoUniTest.py' -- Runtime error: Line 32

Solution 1:

Direct citation from the helpfile (Using PySide):

Normally one creates a PySide application object in a script using QtGui.QApplication(). However, in 3ds Max, there is already a PySide application running, so you get a handle for that object like this:

QtGui.QApplication.instance()

Solution 2:

As a note this has changed somewhat in 3DS Max 2018 and PySide2. I'm just playing around with it myself right now and I was able to get it working after a bit of tinkering. Here's a link to the documentation, though be warned that there is a small typo in the code (at least at the time of writing): http://help.autodesk.com/view/3DSMAX/2018/ENU/?guid=__developer_what_s_new_in_3ds_max_python_api_what_s_new_in_the_3ds_max_2018_p_html

As mentioned in other answers you need to make your UI a child of the main 3DS Max application. The good news is that they have simplified this a bit for you with the function GetQMaxMainWindow(). Use it like this:

from PySide2 import QtWidgets, QtCore, QtGui
import MaxPlus
import os

classSampleUI(QtWidgets.QDialog):
    def__init__(self, parent=MaxPlus.GetQMaxMainWindow()):
        super(SampleUI, self).__init__(parent)
        self.initUI()

    definitUI(self):
        mainLayout = QtWidgets.QHBoxLayout()
        testBtn = QtWidgets.QPushButton("Test!")
        mainLayout.addWidget(testBtn)
        self.setLayout(mainLayout)

if __name__ == "__main__":
    try:
        ui.close()
    except:
        pass

    ui = SampleUI()
    ui.show()

Solution 3:

You're creating an instance of QApplication in the line:

app = QApplication(sys.argv)

And getting that error because there's another instance of QApplication created somewhere before that (presumably somewhere in "3Dsmax 2015 Listener") and you're only allowed one.

See:

QT documentation on QApplication