Joining table field with shapefile using PyQGIS

Question

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')
QgsMapLayerRegistry.instance().addMapLayer(Heads)
self.iface.legendInterface().setLayerVisible(Heads,False)
grid = ftools_utils.getMapLayerByName(unicode('Grid'))
joinObject = QgsVectorJoinInfo()
joinObject.targetFieldName = str('ID')
joinObject.joinLayerID = Heads.id()
joinObject.joinFieldName = str('cellID')
grid.addJoin(joinObject)
grid.reload()

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...

joinObject.setJoinFieldNamesSubset(['someFieldName'])

Have you read Joining table to shapefile with pyQGIS already? — Germán Carrillo, Feb 08 '15 at 13:13
indeed, this as well http://gis.stackexchange.com/questions/67062/pyqgis-joining-a-dbf-to-a-shapefile — Katalpa, Feb 08 '15 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 '15 at 11:49

score 9 · Accepted Answer · answered Feb 08 '15 at 15:07

9

This has worked for me:

# Get input (csv) and target (Shapefile) layers
shp=iface.activeLayer()
csv=iface.mapCanvas().layers()[0]

# Set properties for the join
shpField='code'
csvField='codigo'
joinObject = QgsVectorJoinInfo()
joinObject.joinLayerId = csv.id()
joinObject.joinFieldName = csvField
joinObject.targetFieldName = shpField
shp.addJoin(joinObject)

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?

answered Feb 08 '15 at 15:07

Germán Carrillo

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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 08 '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? – Luigi Pirelli Mar 11 '15 at 11:48
@LuigiPirelli addJoin returns True, giving no clue about any error or typo. – Germán Carrillo Mar 12 '15 at 16:25
1

There's also an excellent tutorial using this method. – chip Aug 22 '16 at 15:54
Keep in mind that: the class QgsVectorJoinInfo has been renamed to QgsVectorLayerJoinInfo. – Taras Jan 26 '22 at 07:11

score 5 · Answer 2 · answered Feb 08 '15 at 13:44

5

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")
QgsMapLayerRegistry.instance().addMapLayer(layer)

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

answered Feb 08 '15 at 13:44

Germán Carrillo

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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 08 '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. – Germán Carrillo Feb 08 '15 at 14:48

score 0 · Answer 3 · edited Feb 21 '17 at 21:08

0

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")
QgsMapLayerRegistry.instance().addMapLayer(layer)

edited Feb 21 '17 at 21:08

PolyGeo

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answered Feb 21 '17 at 20:41

Fidel Lopez Salazar

1
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Joining table field with shapefile using PyQGIS

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