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?

I'm confused: Why was my code 'edited' without being changed in any way? — i.i.k., Jun 10 '23 at 22:35
Editing is a normal part of these sites. In the above case, the tag pyqgis was added to the backticks to tell it how to colourize it. — Rohit Gupta, Jun 14 '23 at 13:26

BERA · Accepted Answer · 2023-06-10T08:18:45.430

7

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)

edited Jun 10 '23 at 08:18

answered Jun 09 '23 at 08:46

BERA

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Oh, this looks very nice, kind of elegant! Thank you very much, I will try it! – i.i.k. Jun 09 '23 at 09:25
I adjusted my code accordingly and ran into a little problem: Python interpretes the value 0 as NULL, so that all values which are 0 or NULL become -999. However, 0 is 0 and not 'None'. Any idea how to tackle this problem? – i.i.k. Jun 11 '23 at 17:32
Oh. Try replacing f2 or f1 or -999 with next(item for item in [f2, f1, -999] if item is not None) – BERA Jun 12 '23 at 06:49

Taras · Answer 2 · 2023-06-14T10:33:59.113

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)')

References:

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

Merging values from two fields into third field using PyQGIS

2 Answers2

refer to a layer by its name

set up an expression including two desired fields

define the expression context to encapsulate parameters for evaluation

start layer editing

Linked