PyQGIS: Multiply fields and populate new field

Question

I am trying to populate field "L_M" with the default value of "Length" or with the result of multiplication between "Length" and 0.3048 if "LengthUnit" = 2, which stands for Feets.

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface
from qgis.PyQt.QtCore import QVariant

# create layer
layer = iface.mapCanvas().currentLayer() 
it = layer.getFeatures(QgsFeatureRequest())


layer.startEditing()

# add field

new_Field1 = "L_M"
idq = layer.fieldNameIndex(new_Field1)

if idq == -1:
    layer.addAttribute(QgsField(new_Field1, QVariant.Double, 'double', 20, 4))

layer.updateFields() # tell the vector layer to fetch changes from the provider

idl = layer.fieldNameIndex('Length')
e = QgsExpression ("idl * 0.3048")
e.prepare(layer.pendingFields())

for feat in it:
    if feat['LengthUnit'] == 2:
        layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('L_M'), e.evaluate(feat))
    else:
        layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('L_M'), idl)

layer.updateFields() # tell the vector layer to fetch changes from the provider

The result I get is 'L_M' field populated with '10.000' value, and NULL value for entries with 'LengthUnit' = 2.

What is the best approach to get the multiplication between fields in a new field?

P.S. As I see it, my script doesn't even read proper values from the 'Length' field, which I don't know why. It's a Double type field also.

JGH · Accepted Answer · 2018-06-04T14:35:01.597

1

The QGSExpression considers the column name, not the column ID. So you would need to change your expression to

e = QgsExpression ('"Length"* 0.3048')

Then in the loop, the else case should set the value of the feature's length, not of the field ID

    else:
        layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('L_M'), feat['Length'])

edited Jun 04 '18 at 14:35

answered Jun 04 '18 at 11:18

JGH

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Perfect @JGH. Many thanks for pointing that out. Another thing was to make the field name escape: e = QgsExpression (""Length" * 0.3048") – kidd Jun 04 '18 at 13:04
@kidd ah yes, I have updated the answer – JGH Jun 04 '18 at 14:35

PyQGIS: Multiply fields and populate new field

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