Adding new Field and writing values for all features

Question

I need to create new Field in Attribute table of vector layer, and afterwards to calculate values and store them in that Field for all features I have in that layer. I have solution which works, but thing is that it is too slow. Does anyone has an idea for faster way?

    # Dodavanje nove kolone
    name = "Vrednost" 
    provider = trecilayer.dataProvider()
    caps = provider.capabilities()
    # Provera da li kolona vec postoji, vraca "-1" ako ne postoji
    ind = provider.fieldNameIndex(name)
    try:
        if ind == -1:
            if caps & QgsVectorDataProvider.AddAttributes:
                res = provider.addAttributes( [ QgsField(name,QVariant.Double) ] )           
    except:
        return False
    #trecilayer.updateFields()

    # Zamisljam da sam koeficijente ucitao iz ulaznog fajla
    #koef=[1.00, 0.85, 0.80, 0.75, 0.70, 0.60, 0.50, 0.40, 0.00]
    lista=open(self.putanja,'r')
    lista.read
    for string in lista:
        x=string.split(",")
    koef=[]
    for i in range(len(x)):
        koef.append(float(x[i]))
    lista.close()



    #priprema za progress bar
    #ukupan broj redova 
    trecilayer.selectAll()
    count = trecilayer.selectedFeatureCount()
    i=0
    #racunanje vrednosti
    trecilayer.startEditing()
    for feature in trecilayer.getFeatures():
        QgsMessageLog.logMessage(str(trecilayer.fieldNameIndex('Vrednost')), 'Vrednost_Parcele')
        razred=feature['Procembeni']
        povrsina=feature.geometry().area()
        nova=povrsina*float(koef[int(razred-1)])
        feature['Vrednost'] = nova
        trecilayer.updateFeature(feature)
        #progress bar
        self.pb(i,count)
        i=i+1

    trecilayer.commitChanges()

Answer 1

You're making your updates feature by feature. I've found that making changes in bulk is orders of magnitude faster.

prepare your changes in the loop by making a dictionary of fids to change and the desired attribute changes. Then run the update outside the loop in a single step as per the Modify Features section of the pyQGIS cookbook Documentation QGIS testing: Using Vector Layers

You may be able to further speed up the code by running a select query to make the loop smaller as per the "Iterating over a subset of features" section