from qgis.core import *
from qgis.utils import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtCore import QFileInfo, QSettings

QgsApplication.setPrefixPath('/Applications/QGIS-1.9.app/Contents/MacOS', True)
QgsApplication.initQgis()

province = QgsVectorLayer('/filepath/Province_studyUTM36S.shp' , 'layer', 'ogr')
town = QgsVectorLayer('/filepath/TownPolygons_UTM36S.shp' , 'layer', 'ogr')
road = QgsVectorLayer('/filepath/Roads_UTM36S.shp' , 'layer', 'ogr')

QgsMapLayerRegistry.instance().addMapLayer(province)
QgsMapLayerRegistry.instance().addMapLayer(road)
QgsMapLayerRegistry.instance().addMapLayer(town)

rasterFile = '/filepath/Landsat.tif'
fileInfo = QFileInfo(rasterFile)
baseName = fileInfo.baseName()
rlayer = QgsRasterLayer(rasterFile, baseName)


QgsMapLayerRegistry.instance().addMapLayer(rlayer)

img = QImage(QSize(800,600), QImage.Format_ARGB32_Premultiplied)

color = QColor(255,255,255)
img.fill(color.rgb())

p = QPainter()
p.begin(img)
p.setRenderHint(QPainter.Antialiasing)
render = QgsMapRenderer()
ID = [  rlayer.getLayerID(), town.getLayerID(), road.getLayerID(), province.getLayerID()]
render.setLayerSet(ID)


rect = QgsRectangle(render.fullExtent())
rect.scale(1.1)
render.setExtent(rect)

render.setOutputSize(img.size(), img.logicalDpiX())

render.render(p)
p.end()

img.save("/filepath/first_render.png","png")

qgis.utils.iface.actionZoomToSelected().trigger()
qgis.utils.iface.mapCanvas().zoomScale(1000)

qgis.utils.iface.mapCanvas().saveAsImage('test.png', None, 'PNG')

zoomRectangle = QgsRectangle(pos[0]-offset, pos[1]-offset,pos[0]+offset,pos[1]+offset)
self.canvas.setExtent(zoomRectangle)
self.canvas.refresh()

self.canvas.setExtent(rect)

self.canvas = qgis.utils.iface.mapCanvas()

self.canvas = self.iface.mapCanvas()

# load the project, load project instance (pi)
pi = QgsProject.instance()
pi.read(project_path)

# get the project's layout
plm = pi.layoutManager()
layout = plm.layoutByName("my_layout") # your layout name

#get reference map
refmap = layout.referenceMap()

xmin = ...you
ymin = ...provide
xmax = ...these
ymax = ...

# set extent
bb = QgsRectangle(xmin, ymin, xmax, ymax )
refmap.setExtent(bb)