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The following is currently unsuccessful at fulfilling a table field join to a point shapefile in Qgis.
All instances of the join object will print the proper values, and no error is flagged when processing - but the target layer does not exhibit any effect.

Something missing?

    Heads = QgsVectorLayer(headCSV, 'Heads', 'ogr')
    grid = ftools_utils.getMapLayerByName(unicode('Grid'))
    joinObject = QgsVectorJoinInfo()
    joinObject.targetFieldName = str('ID')
    joinObject.joinLayerID =
    joinObject.joinFieldName = str('cellID')


The following returns 'None'. I expect this is actually supposed to return the field names to be joined? (e.g. - a clue to the mistake)

    print joinObject.joinFieldNamesSubset()


Added the following, and now the previous update command returns the accurate fields to be joined - yet the destination layer does not show 'joined' fields...

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Have you read Joining table to shapefile with pyQGIS already? – gcarrillo Feb 8 at 13:13
indeed, this as well… – Katalpa Feb 8 at 13:18
sould be noted that join works on layer added in the registry. Otherwise no update fields would happen. – Luigi Pirelli Mar 11 at 11:49
@Luigi, Thanks for clarification! – Katalpa Mar 11 at 12:32

2 Answers 2

up vote 2 down vote accepted

This has worked for me:

# Get input (csv) and target (Shapefile) layers

# Set properties for the join
joinObject = QgsVectorJoinInfo()
joinObject.joinLayerId =
joinObject.joinFieldName = csvField
joinObject.targetFieldName = shpField

The error you are facing is, believe it or not, by a typo in joinLayerId. The question is, why QGIS doesn't throw an error there?

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You are exactly right. Works beautifully now. Highly appreciate the help. A mistake easily overlooked it seems without a syntax error. Thanks! – Katalpa Feb 8 at 15:10
@gcarrillo "The question is, why QGIS doesn't throw an error there?" because your are creating a new python variable. Did you check if addJoin return false? – Luigi Pirelli Mar 11 at 11:48
@LuigiPirelli addJoin returns True, giving no clue about any error or typo. – gcarrillo Mar 12 at 16:25

If it's ok for you to use Processing algorithms for that, it's as simple as this (from the QGIS Python console):

import processing
res = processing.runalg("qgis:joinattributestable","/path/to/shape.shp","/path/to/table.csv","fieldShp","fieldTable","/path/to/output.shp")
layer = QgsVectorLayer(res['OUTPUT_LAYER'], "joined layer", "ogr")

You can have a look at the algorithm source code so that you decide whether you want to adapt it for your own script.

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While this is a great alternative, the need is a 'virtual join', as accomplished from the properties menu of a layer. This is because the operation might be done repeatedly, and avoidance of data overhead is a must. – Katalpa Feb 8 at 13:51
Well, you could still use temporary layers setting the last parameter of the algorithm to None, but I see your point. – gcarrillo Feb 8 at 14:48

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