Merging values from two fields into third field using PyQGIS

Question

I have two fields which I want to merge into a single field, thereby should one of the two to-be-merged fields be prioritized, meaning, if a value of one field is available take it instead of the other field.

I would like to have it like this:

field1 | field2 | resultField
-----------------------------
13     | NULL   | 13
28     | 56     | 56
44     | NULL   | 44
11     | 17     | 17

So the "field2" should be prioritized in the resulting field.

I know, that I will need to create a new field, with the name relevant field name, so

vectorLyr =  QgsVectorLayer('path_to_filed', 'layername', "ogr")
vectorLyr.isValid()

vpr = vectorLyr.dataProvider()

fieldnames = [resultField]

for name in fieldnames:
    vpr.addAttributes([QgsField(name, QVariant.Double)])
    vectorLyr.updateFields()

However, how can I now update the "resultfield" with the values from the two fields?

Answer 1

You can use or like this, i think it is called Ternary operators:

layer = iface.activeLayer()
p = layer.dataProvider()
field_to_update = "field3"
fieldindex = layer.fields().indexOf(field_to_update)

attributemap = {} #A dictionary to store {feature id: {index of field to update : new value}}
for f in layer.getFeatures(): #For each feature
    f1, f2 = f["field1"], f["field2"] #Extract the values in field1 and field2
    attributemap[f.id()] = {fieldindex: f2 or f1 or -999} #Take the first not null value, or if there is none, use -999

#attributemap[1]
#{4: 83} Feature 1 should in field 4 get the value 83

p.changeAttributeValues(attributemap)

Answer 2

Another approach might be not as Pythonic as was suggested by @BERA, however, it is still applicable in terms of PyQGIS.

It utilizes the coalesce() function inside the QgsExpression() class.

Proceed with Plugins > Python Console > Show Editor and paste the script below

from qgis.core import QgsProject, QgsExpression, QgsExpressionContext, QgsExpressionContextUtils

# refer to a layer by its name
layer = QgsProject.instance().mapLayersByName("lines")[0]

# set up an expression including two desired fields
expression = QgsExpression('coalesce("field1", "field2")')

# define the expression context to encapsulate parameters for evaluation
context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(layer))

# start layer editing
with edit(layer):
    for feat in layer.getFeatures():
        context.setFeature(feat)
        feat["result"] = expression.evaluate(context)
        layer.updateFeature(feat)

Press Run script and get the output that will look like:

P.S. To achieve -999 where "field1" and "field2" are both NULLs, please modify the expression accordingly:

expression = QgsExpression('coalesce("field1", "field2", -999)')

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References:

• PyQGIS Developer Cookbook | 11. Expressions, Filtering and Calculating Values