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 = Heads.id()
    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...


This has worked for me:

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

# Set properties for the join
joinObject = QgsVectorJoinInfo()
joinObject.joinLayerId = csv.id()
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?

  • 1
    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 '15 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? Mar 11 '15 at 11:48
  • @LuigiPirelli addJoin returns True, giving no clue about any error or typo. Mar 12 '15 at 16:25
  • 1
    There's also an excellent tutorial using this method.
    – chip
    Aug 22 '16 at 15:54

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.

  • 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 '15 at 13:51
  • Well, you could still use temporary layers setting the last parameter of the algorithm to None, but I see your point. Feb 8 '15 at 14:48

Excellent example, I was very useful, combine a shp with a csv file, here my example performed:

import processing
res = processing.runalg("qgis:joinattributestable","d:/areas_geoestadisticas_estatales.shp","d:/poblacion.csv","cve_ent","cve_ent","d:/union_shp_csv.shp")
layer = QgsVectorLayer(res['OUTPUT_LAYER'], "Capa Unida", "ogr")

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