Iterate through list of field names and calculate fields with PyQGIS?

Question

I am using QGIS 3.2.1 Bonn 64 bit on Windows 10. I would like to loop through a list of field names and perform calculations on each field. Only the first loop in the iteration performs any calculations. The rest are skipped.

result_path = r"D:\QGIS\hex_results.geojson"

tracks = QgsProject.instance().mapLayersByName("patrol_tracks")[0]
result = QgsProject.instance().mapLayersByName("hex_results")[0]

t = tracks.getFeatures()
r = result.getFeatures()

r_prov = result.dataProvider()
r_fields = r_prov.fieldNameMap()

months = [x for x in processing.run("qgis:listuniquevalues", {'INPUT':tracks,'FIELDS':"Month_Year",'OUTPUT':'u'})['UNIQUE_VALUES'].split(";")]

for m in months:
    result.startEditing()
    field_index = result.fields().lookupField(m)
    print (m)
    print (field_index)
    a = {field_index : 0}
    for feat in r:
        fid = feat.id()
        r_prov.changeAttributeValues({fid : a})

    result.commitChanges()

The print calls work for every list item but the field attributes are only altered for the first iteration.

Another method I have tried (using the same variable names as above) is:

for m in months:
    processing.run('qgis:fieldcalculator', {'INPUT':result, 'FIELD_NAME':m, 'FIELD_TYPE':1, 'FIELD_LENGTH':10, 'FIELD_PRECISION':0, 'NEW_FIELD': False, 'FORMULA':'0', 'OUTPUT':result_path})

But with the same result - only the first field in the list is calculated. This code will be part of a more complex script that calculates field values based on the intersection of selected features - which is more or less working up until this point.

Any suggestions? I wondered if the dataProvider needs to be refreshed or updated after each iteration but commitChanges(), added later, doesn't seem to have any effect.

Answer 1

The QgsFeatureIterator is closed after returning all features.

Example:

>>layer = iface.activeLayer()
>>feats = layer.getFeatures()

>>[f['GRID_ID'] for f in feats]
['AP7', 'AP8', 'AP9', 'AP3', 'AP4', 'AP5', 'AP6', 'AP1', 'AP2']

>>[f['GRID_ID'] for f in feats]
[] #No more features are return after used once

>>feats = layer.getFeatures() #Recreate it
>>[f['GRID_ID'] for f in feats]
['AP7', 'AP8', 'AP9', 'AP3', 'AP4', 'AP5', 'AP6', 'AP1', 'AP2']

So in your case recreate it for each iteration:

...
for m in months:
    result.startEditing()
    field_index = result.fields().lookupField(m)
    print (m)
    print (field_index)
    a = {field_index : 0}
    for feat in r:
        fid = feat.id()
        r_prov.changeAttributeValues({fid : a})
    result.commitChanges()
    r = result.getFeatures()