[GIS] How to add an attribute with QGIS 2.8.1 Python console

fields-attributespyqgis

i would like to add an attribute with the python console:

from PyQt4.QtCore import *
from PyQt4.QtGui import *
layer = iface.addVectorLayer("C:/donnees_SIG/05.shp","nou","ogr")
layer = iface.activeLayer()
layer.startEditing()
layer.addAttribute(QgsField("STATUT", QVariant.Int))

But it doesn't work…
When i write the last line, the python console returns 'False'.

I'm using QGIS 2.8.1

Best Answer

Look at Using Vector Layers: Adding and Removing Fields

res = layer.dataProvider().addAttributes([QgsField("STATUT", QVariant.Int)])
layer.updateFields()

QGIS 2.0 (and 1.9 / master)

fid = 1 # feature id
attrs = { 0: 2,1: "B",2:"Dont!!!"} # attributes
vLayer.changeAttributeValues({ fid : attrs })

or a very intuitive method, if you already have a copy of the feature you are going to update

fid = 1 # feature id
feat = vLayer.getFeatures( QgsFeatureRequest().setFilterFid( fid ) ).next()
feat['number']  = 2
feat['char']    = 'B'
feat['advice']  = 'Dont!!!'
vLayer.updateFeature( feat )

QGIS 1.8

fid = 1 # feature id
attrs = { 0: QVariant(2),1: QVariant("B"),2:QVariant("Dont!!!")} # attributes
for attr, val in attrs.iteritems():
    vLayer.changeAttributeValue( fid, attr, val )

There is no need to start an edit session if you use the vLayer.dataProvider().insertEditCommandHere() methods, as the edit command will be executed immediately.

(This is a very common misconception and I guess it should be added to the QGIS documentation and pyqgis cookbook more prominently)

[GIS] How to compute an interpolation raster from the python console in QGIS

The documentation on pyqgis is not very self-explanatory, but i figured out how to properly call the associated interpolation classes (QgsInterpolator, QgsTINInterpolator, QgsIDWInterpolator, QgsGridFileWriter) from python. I am going to describe every step of the script in great detail:

Step 1:

Import the core and analysis module and get the desired vector layer for interpolation by selecting it with a mouseclick in the layer tab.

import qgis.core
import qgis.analysis

layer = qgis.utils.iface.activeLayer()

Step 2:

Prepare the interpolation classes with the necessary Parameters. The exact parameters for initialization of the LayerData struct can be found in the QGIS API docs (searchterm: QgsInterpolator).

layer_data = QgsInterpolator.LayerData()
layer_data.vectorLayer = layer
layer_data.zCoordInterpolation=False
layer_data.interpolationAttribute =0
layer_data.mInputType = 1

Please notice that I don't use the z Coordinate, I get the first available field (index = 0) as interpolation attribute, and use POINTS as input type.

Step 3:

Choose your interpolation engine. Here you can choose between the TIN-Interpolation method (QgsTINInterpolator) and IDW-Interpolation (QgsIDWInterpolator). I took the QgsTINInterpolator in my code.

tin_interpolator = QgsTINInterpolator([layer_data])

Keep in mind that you have to pass a python list of layer_data to the interpolation engine! This also allows you to add multiple layer_data scenarios.

Step 4:

Setup the parameters that are needed for the export of the interpolation-output (see documentation of QgsGridFileWriter). Those include similar information as the interpolation gui (filepath, extent, resolution, number of colums and rows).

export_path ="C:/SomeFolder/output.asc"
rect = layer.extent()
res = 10
ncol = int( ( rect.xMaximum() - rect.xMinimum() ) / res )
nrows = int( (rect.yMaximum() - rect.yMinimum() ) / res)

output = QgsGridFileWriter(tin_interpolator,export_path,rect,ncol, nrows,res,res)
output.writeFile(True)  

iface.addRasterLayer(export_path, "interpolation_output")

Be aware of the file extension of your output-raster as QgsGridFileWriter only writes ASCII-grids (.asc). The data gets written to disk by calling the writeFile() method. After export you can add the grid-file as raster to the canvas.

Full script for reference:

import qgis.analysis
import qgis.core

layer = qgis.utils.iface.activeLayer() 
layer_data = QgsInterpolator.LayerData()
layer_data.vectorLayer = layer
layer_data.zCoordInterpolation=False
layer_data.interpolationAttribute =0
layer_data.mInputType = 1


tin_interpolator = QgsTINInterpolator([layer_data])

export_path = "E:/GIS_Workbench/script_output/test.asc"

rect = layer.extent()
res = 10
ncol = int( ( rect.xMaximum() - rect.xMinimum() ) / res )
nrows = int( (rect.yMaximum() - rect.yMinimum() ) / res)
output = QgsGridFileWriter(tin_interpolator,export_path,rect,ncol,nrows,res,res)
output.writeFile(True)

Keep in mind that the QGIS-API is currently rewritten to version 3.0 and the used interpolation-classes are moved from qgis.analysis to qgis.core! This will have a huge impact on the functionality of this script so that it must be rewritten for version 3.0!