Deleting selected features using PyQGIS

Question

I have a shapefile with only one field "DN". I would like to delete all features which does not have specific value of the field. Let's say the "DN" can be 1,2 or 3.

So far I was able to find out how to select Features and how to delete feature, but I did not succeed to combine them. Let's say I want only features with "DN" = 3 to remain:

Selection of the Features found here : Filter features based on their attributes using Python, however, I do not know if it does what it is supposed to do since I did not find the way how to check what is inside.

select = layer.getFeatures(QgsFeatureRequest().setFilterExpression(u'"DN"!=3'))

delete features found in the PyQGIS Developer Cookbook

res = Ilayer.dataProvider().deleteFeatures()

As far as I know, the deleteFeatures() requires "ID"s of the features, but I have no idea how to get them from from select which is QgsFeatureIterator.

Answer 1

You can loop over the iterator and get the id() for every feature in it:

with edit(layer):
    # build a request to filter the features based on an attribute
    request = QgsFeatureRequest().setFilterExpression('"DN" != 3')

    # we don't need attributes or geometry, skip them to minimize overhead.
    # these lines are not strictly required but improve performance
    request.setSubsetOfAttributes([])
    request.setFlags(QgsFeatureRequest.NoGeometry)

    # loop over the features and delete
    for f in layer.getFeatures(request):
        layer.deleteFeature(f.id())

or with QGIS < 2.12

request = QgsFeatureRequest().setFilterExpression('"DN" != 3')
request.setSubsetOfAttributes([])
request.setFlags(QgsFeatureRequest.NoGeometry)

ids = [f.id() for f in layer.getFeatures(request)]

layer.startEditing()
for fid in ids:
    layer.deleteFeature(fid)
layer.commitChanges()

Answer 2

There is also a possibility of using the deleteSelectedFeatures() method from the QgsVectorLayer class for deleting selected features:

# imports
from qgis.utils import iface

# choose an active layer
layer = iface.activeLayer()

# make your own selection
field_name = "DN" # specify your field
field_values = (1,2,3) # specify values for features that you want to retain
expression = f'"{field_name}" in {field_values}'
layer.selectByExpression(expression)

# delete selected features
if layer.selectedFeatureCount() > 0:
    layer.startEditing()
    layer.deleteSelectedFeatures()
    layer.commitChanges()

Answer 3

If run from a script, exiting qgis will solve the issue, that is, including this part in the script:

qgs = QgsApplication([], False)
...
qgs.exitQgis()
os.remove()