layer = iface.activeLayer()
areatot = 0
for elem in layer.getFeatures()
     geom = elem.geometry()
     areatot += geom.area()

aeratot = [sum(elem.geometry().area() for elem in layer.getFeatures())]

# creation of a empty geometry for unioning
geomtot = QgsGeometry.fromWkt('GEOMETRYCOLLECTION EMPTY')
# union
for elem in layer.getFeatures():
   geomtot = geomtot.combine(elem.geometry())
area = geomtot.area()

layer = qgis.utils.iface.activeLayer()
areatot = 0
layer.select()
for elem in layer:
     geom = elem.geometry()
     areatot += geom.area()

geomtot = QgsGeometry.fromWkt('GEOMETRYCOLLECTION EMPTY')
layer.select()
for elem in layer:
    geomtot = geomtot.combine(elem.geometry())
area = geomtot.area()

vlayer = QgsVectorLayer("/path/to/shapefile/file.shp", "layer_name_you_like", "ogr")

vlayer.extent().toString()