Iterating and calculating with fields using PyQGIS

Question

Inside my layer there are fields like this:

name | data | length
name_a | field_a_x | ...
name_b | field_b_x | ...
name_c | field_c_x | ...
name_d | field_d_x | ...

Is it possible to iterate only over the data-column, get the fields into a list and replace the data after calculating (or doing something else...)? The calculating-part will be easy but I'm having trouble understanding how to get and afterwards replace the data?

This is what I got so far...

from qgis.core import QgsProject
layer_name = "ExampleLayer"
layer = QgsProject.instance().mapLayersByName(layer_name)[0]
ftrs = layer.getFeatures()
list_old = []
for feat in layer.getFeatures():
 list_old.append(feat['data'])

• Corentin: hello, can you add the result you are waiting for or an example of expression you would like to use ?
• finethen: @CorentinLemaitre The new list could be anything actually! My problem is how to get the data into the field (updating)

Answer 1

One example:

lyr = iface.activeLayer()
feats = [f for f in lyr.getFeatures()] #List all features
#names = [f['KOMMUNNAMN'] for f in feats] #List values in KOMMUNNAMN field
#['Kiruna', 'Pajala', 'Gällivare', 'Jokkmokk', 'Boden']
fieldindex = lyr.fields().indexFromName('KOMMUNNAMN')
attrMap = {}
for f in feats:
 attrMap[f.id()] = {fieldindex:f['KOMMUNNAMN'].upper()}
#attrMap
#{0: {3: 'KIRUNA'}, 1: {3: 'PAJALA'}, 2: {3: 'GÄLLIVARE'}, 3: {3: 'JOKKMOKK'}, 4: {3: 'BODEN'}}
lyr.dataProvider().changeAttributeValues(attrMap)

enter image description here

• finethen: Thank you very much for your help!