from PyQt4.QtCore import *

import qgis

from qgis.core import QgsMapLayerRegistry, QgsVectorJoinInfo

for layer in QgsMapLayerRegistry.instance().mapLayers().values():

# If a layer name = postcode, define it as a variable called 'shp'

if layer.name() == "postcode":

qgis.utils.iface.setActiveLayer(layer)

shp = qgis.utils.iface.activeLayer()

for layer in QgsMapLayerRegistry.instance().mapLayers().values():

# If a layer name = CRVsignups, define it as a variable called 'csv'

if layer.name() == "CRVsignups":

qgis.utils.iface.setActiveLayer(layer)

csv = qgis.utils.iface.activeLayer()

# Set up join parameters

shpField='pcode'

csvField='pcode'

joinObject = QgsVectorJoinInfo()

joinObject.joinLayerId = csv.id()

joinObject.joinFieldName = csvField

joinObject.targetFieldName = shpField

shp.addJoin(joinObject)

# Define fields to update and joined fields to copy values from

ip1 = shp.fieldNameIndex('Total')

ip1_join = shp.fieldNameIndex('CRVsignups_Total')

shp.startEditing()

for feat in shp.getFeatures():

shp.changeAttributeValue(feat.id(), ip1, feat.attributes()[ip1_join])