[QGIS][2.x] Example of iterating layers, attributes and features

layermap = QgsMapLayerRegistry.instance().mapLayers()
   
for (name,layer) in layermap.iteritems():
    if layer.type() != QgsVectorLayer.VectorLayer:
        continue
     
    if "SampleLayer" in layer.name():
        dP = layer.dataProvider()
     
        for attr in dP.fields():
            if (attr.name() == "SampleAttr"):
                SampleAttrId = dP.fieldNameIndex(attr.name())
                break
 
 
    layer.select(dP.attributeIndexes())
     
    FetSet = layer.getFeatures()
 
    for feat in FetSet:
        SampleAttrStr = str(feat[SampleAttrId])
        SampleAttrValue = float(str(feat[SampleAttrId]))