Code to reproduce it quickly in a Python module:
class MyWidget:
def init(self, parent = None):
if not parent:
self.parent = slicer.qMRMLWidget()
self.parent.setLayout(qt.QVBoxLayout())
self.parent.setMRMLScene(slicer.mrmlScene)
else:
self.parent = parent
self.layout = self.parent.layout()
if not parent:
self.setup()
self.parent.show()

from functools import partial
def __onNodeAddedObserver__(self, caller, eventId, callData):
  """Node added to the Slicer scene"""
  if callData.GetClassName() == 'vtkMRMLScalarVolumeNode':
    print ("Loaded " + callData.GetID())

self.__onNodeAddedObserver__ = partial(__onNodeAddedObserver__, self)
self.__onNodeAddedObserver__.CallDataType = vtk.VTK_OBJECT
print ("adding observer")
slicer.mrmlScene.AddObserver(slicer.vtkMRMLScene.NodeAddedEvent, self.__onNodeAddedObserver__)