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?

Answer 1

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

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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

• user99: 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.
• Germán Carrillo: I've edited the answer to let you know how to access Shapefiles from a directory.
• Germán Carrillo: 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.
• user99: I am working with a stand alone script where i want to join the attribute tables and only the selected fields.

Answer 2

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)