Selecting from selection with PyQGIS

Question

The expression box has a handy "select from selection" button. Is there a way to emulate this with Python? I have the basic expressions down, but can't find a way to do this.

Answer 1

I didn't find the option from the API, but you could mimic such behavior this way:

I assume there are selected features already, so get their Ids:

lyr = iface.activeLayer()
selIds = lyr.selectedFeaturesIds()

You said you have an expression, let's say:

expr = QgsExpression( "\"NMG\" = 'CALI'" )

Now, get feature Ids that match your expression:

it = lyr.getFeatures( QgsFeatureRequest( expr ) )
newIds = [i.id() for i in it]

Get only those that are already selected and match your expression (this is the magic part):

idsToSel = list( set(selIds).intersection(newIds) )

Finally, select them:

lyr.setSelectedFeatures( idsToSel )

Answer 2

To select from the selected objects (filter the current selection) in QGIS 3, I used IntersectSelection, it Modify current selection to include only select features which match.

layer = self.iface.activeLayer()
myExpression = '\"population\" > 100'
layer.selectByExpression(myExpression, QgsVectorLayer.IntersectSelection)

From the current selection, only objects with a population greater than 100 will remain selected. If there was no current selection, then no objects will be selected.