Load a specific spatial data from a postGIS table

Question

A single postGIS table contains multiple type of spatial data(example: Linestring,Polygon,etc.,). How to load a specific spatial data from PostGIS table into QGIS by using pyqgis?

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Sample code snippet is given below

uri = QgsDataSourceURI()    
uri.setConnection("xxx.xxx.xxx.xxx", "port", "dbname", "username", "password")                
uri.setDataSource("public","tablename","geom")

uri.setWkbType(QGis.WKBPolygon) #geometry type has to be mentioned

uri.setSrid('4326')
vlay=QgsVectorLayer(uri.uri(),"test_poly","postgres")
QgsMapLayerRegistry.instance().addMapLayer(vlay)