How to join the attributes table from one input layer to another layer with unique id using python

Question

I have one input layer with the attibute values and i wanted to join the attribute table of the input layer to the another layer with unique values using python. gis.stackexchange.com/questions/89706/how-to-merge-shape-files-with-attributes-from-a-python-script-in-qgis This link gives the information about about how to merge the features and attributes manually. but i want to join the atttribute tables. How can i do that using python?

Germán Carrillo · Accepted Answer · 2015-07-28T13:59:04.507

2

You can join your two layers in this way:

# Get input and target layers
targetLyr = iface.mapCanvas().layers()[1] # Second layer from the ToC
inLyr = iface.mapCanvas().layers()[0] # First layer from the ToC

# Set properties for the join
targetField='myTargetField'
inField='myInputField'
joinObject = QgsVectorJoinInfo()
joinObject.joinLayerId = inLyr.id()
joinObject.joinFieldName = inField
joinObject.targetFieldName = targetField
targetLyr.addJoin(joinObject) # You should get True as response

EDIT: If you don't have your layers loaded into QGIS, you can access them from a directory in this way:

targetLyr = QgsVectorLayer('/path/to/layer1.shp', 'Layer 1', 'ogr') # I assume you use Shapefiles
inLyr = QgsVectorLayer('/path/to/layer2.shp', 'Layer 2', 'ogr') # First layer from the ToC

edited Jul 28 '15 at 13:59

answered Jul 27 '15 at 14:16

Germán Carrillo

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Thank you. The method works only if the files are opened in the TOC. But i dont want to open the input layer in the TOC and i want to take it from a directory. – user99 Jul 28 '15 at 07:36
I've edited the answer to let you know how to access Shapefiles from a directory. – Germán Carrillo Jul 28 '15 at 14:00
One thing that is still unclear from your question is whether you're going to work inside QGIS (i.e., using the QGIS Python console), or with a standalone script (where you don't need to open QGIS at all). Please let me know about it, so that I can make my answer more to the point. – Germán Carrillo Jul 28 '15 at 14:18
I am working with a stand alone script where i want to join the attribute tables and only the selected fields. – user99 Jul 29 '15 at 05:12

score 1 · Answer 2 · edited Apr 13 '17 at 12:33

If you want to use a solution with processing as in How to merge shape files with attributes from a python script in QGIS? (without in the solution of gcarillo):

1) The easiest solution is to is to run the wanted algorithm from the toolbox and look at the /.../.qgis2/processing/processing.log file. The last lines gives the solution:

ALGORITHM|Mon Jul 27 2015    
16:29:33|processing.runalg("qgis:joinattributesbylocation","/Users/Shared/test.shp","/Users/Shared//test2.shp","['intersects', 'contains', 'equals', 'overlaps', 'within']",0,"sum,mean,min,max,median",1,None)

2) For the explanation, look at Using processing algorithms from the console:

import processing
processing.alglist("join")
Join attributes by location-------------------------->qgis:joinattributesbylocation
Join attributes table-------------------------------->qgis:joinattributestable

and for the parameters:

processing.alghelp("qgis:joinattributesbylocation")
....
processing.alghelp("qgis:joinattributestable")
....

With an example and comments in Overlay Analysis in QGIS (or Preferably Python)

How to join the attributes table from one input layer to another layer with unique id using python

2 Answers2