Creating new field with floating values from already existing string field using PyQGIS

Question

Iт QGIS 3.10 I have imported a CSV which contains a column of numeric data (let's call this attribute "x"). However, it is imported automatically as strings. I can easily create a new field by clicking the Field Calculator and typing to_real("x").

Because I need to repeat this many times, I would like to use PyQGIS to do this.

I've been trying various ways but I still can't get it to work. For example, I was following this: Struggling to use Field Calculator - PyQGIS

Could someone please tell me what I did wrong?

#import data
csv_file = QgsVectorLayer(path_to_csv, 'nameoflayer')
QgsProject.instance().addMapLayer(csv_file)

csv = QgsProject.instance().mapLayersByName('nameoflayer')[0]

#start editing this layer
with edit(csv):
    for feature in selected_feature:
        feature.setAttribute(feature.fieldNameIndex('newfieldname'), to_real(feature['x']))
        updateFeature(feature)

There is no error message but I can't see the new column called "newfieldname" when I open the attribute table after running the above code.

Answer 1

to_real is a function belonging to Field Calculator. Its Python equivalent is float. I assume "newfieldname" already exists in the attribute table. Use this script:

csv = QgsProject.instance().mapLayersByName('nameoflayer')[0]

with edit(csv):
    csv.addAttribute(QgsField('newfieldname', QVariant.Double))

    for feature in csv.getFeatures(): # OR csv.selectedFeatures():
        feature['newfieldname'] = float(feature['x'])
        csv.updateFeature(feature)