Sorting/ordering selected features by attribute in QgsVectorLayer using PyQGIS

Question

In a standalone application, I have a layer to which I select some features. Then I need to do an iteration, and I need it to be done in the order of the value of a field/attribute (In my example order by Name of the Cities).

vLayer = QgsVectorLayer(cities_of_world_file, 'Cities of World', "ogr")
country = 'Spain'
filter = '"Country"=\'%s\' '%country
vLayer.selectByExpression(filter, QgsVectorLayer.SetSelection)
for feature in vLayer.selectedFeatures():
    print(feature['City'])

I think there are two ways:

1. Open the layer ordered, and after that, you make a selection. In SQL when you select records you can say ORDER BY, is this order in a QgsVectorLayer?

2. Select features and after that, you sort.

How can I do this?

Answer 1

You can also use addOrderBy method of QgsFeatureRequest in this way:

request = QgsFeatureRequest().addOrderBy("City")
for f in vLayer.getSelectedFeatures(request):
    print(f["City"])

For a large dataset, using selectedFeatures() may take a long time, Instead, you may consider using the way in this answer.

getSelectedFeatures() is more memory-friendly when handling large selections.

Answer 2

Try the snippet below. You can store selected features and an associated attribute as key, value pairs into a dictionary, then sort by the field value. After that, you can access the feature object stored as the key.

vLayer = QgsVectorLayer(cities_of_world_file, 'Cities of World', "ogr")

country='Spain'
filter='"Country"=\'%s\' '%country
vLayer.selectByExpression(filter,QgsVectorLayer.SetSelection)

selected = {f: f['City'] for f in vLayer.selectedFeatures()}
sorted_by_val = {k: v for k, v in sorted(selected.items(), key=lambda item: item[1])}

for feat, val in sorted_by_val.items():
    print(val)
    # do something with feat

https://stackoverflow.com/questions/613183/how-do-i-sort-a-dictionary-by-value

Answer 3

This also works:

sorted_list = sorted(vLayer.selectedFeatures(), key=lambda el: el['City'])

for feature in sorted_list:
    print(feature['City'])

Answer 4

@Ben W, thank you so much, I have marked your answer as correct.

Even so I have found a solution to solve it in one operation. It is a combination of your answer and the instructions on this website to sort a list with a dictionary inside(QgsVectorLayer.selectedFeatures()):

https://www.w3schools.com/python/ref_list_sort.asp

vLayer= QgsVectorLayer(cities_of_world_file, 'Cities of World', "ogr")
country='Spain'
filter='"Country"=\'%s\' '%country
vLayer.selectByExpression(filter,QgsVectorLayer.SetSelection)

iter=vLayer.selectedFeatures() # Is a list with a like dictionary inside
iter.sort(key=lambda item: item['City'])

for feature in iter:
    print(feature['City'])

Do you Know if it's possible to do my first way:

1.- Open Layer ordered, and after that you make selection. In SQL when you select records you can say ORDER BY, exist this order in a QgsVectorLayer?