qgis - Why does iface.activeLayer() not work in a PyQGis standalone app?

It is my second day on PyQGIS. Not sure why layer doesn't return anything. Can anyone help me on this?

Code:

#!/usr/bin/python
from qgis.core import *
from qgis.utils import *

from PyQt4.QtCore import *

def main():
  qgis_prefix="/usr"    
  QgsApplication.setPrefixPath(qgis_prefix, True) 
  QgsApplication.initQgis()
  app = QgsApplication([], False)

  uri = "elevp.csv?delimiter=%s&xField=%s&yField=%s&elevField=%s" % (";","x","y","elev")
  vlayer = QgsVectorLayer(uri, "test", "delimitedtext")

  QgsMapLayerRegistry.instance().addMapLayer(vlayer)
  #print(QgsMapLayerRegistry.instance().mapLayers())


  #iface.mapCanvas().setSelectionColor( QColor("red") )
  layer = iface.activeLayer()
  print(layer)


# Run main wrapper codes

main()