Quick pattern of how to enforce UI singletons.

pyside_ui_singleton_example.py

# -*- coding: utf-8 -*-
import logging

from PySide import QtGui
from PySide import QtCore


def delete_instances(widget_type):
    """Search for all instances of widget_type and delete them. We find them
    by class name instead of type since this is safer with all the reload() action
    used in DCC interpreter sessions.
    """

    widgets = [wdgt for wdgt in QtGui.QApplication.allWidgets()
               if(widget_type.__name__ == type(wdgt).__name__)]

    for wdgt in widgets:
        wdgt.deleteLater()


class UISingleton(QtGui.QWidget):

    def __new__(cls, *args, **kwargs):
        """Delete and cleanup old instances."""

        delete_instances(cls)
        return super(UISingleton, cls).__new__(cls, *args, **kwargs)

    def __init__(self, parent=QtGui.QApplication.activeWindow()):
        super(UISingleton, self).__init__(parent=parent)

        name = "{0}.{1}".format(self.__module__, type(self).__name__)
        self.logger = logging.getLogger(name)

        self.setWindowTitle("UISingleton")
        self.setWindowFlags(QtCore.Qt.Window)
        self.show()


if(__name__ == "__main__"):
    UISingleton()