failed_singleton_widget.py

from PySide import QtGui

_testwidget_instance = None

class TestWidget(QtGui.QLabel):
    def __init__(self, parent=None):
        super(TestWidget, self).__init__(parent)

        # There can be only one.. assuming this works..
        global _testwidget_instance
        if _testwidget_instance is None:
            _testwidget_instance = self
        else:
            # Ideally the panel would close itself here, but
            # can't work out how to access the PythonPanel
            # instance from here (nuke.thisPane is close,
            # but nope)
            nuke.message("Panel already open, stopit")
            self.setText('Was already open')
            return

        # Regular panel'y stuff
        self.setObjectName('myID')
        self.setText('only show me once')

    def hideEvent(self, evt):
        global _testwidget_instance
        _testwidget_instance = None


nukescripts.registerWidgetAsPanel('TestWidget', 'test widget', 'myID')