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.

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    Something unexpected came up so I had to deal with that first.. but I tested your solution just now. Works great! – MapEngine Mar 3 '15 at 19:34
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    You could try to use your ids with request.setSubsetOfAttributes . – Matte Sep 4 '17 at 9:33
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    For anyone testing this on QGIS < 2.99, they should use ids = vl.selectedFeaturesIds() instead :) – Joseph Sep 4 '17 at 9:49
  • @Matte do you suggest to use request.setSubsetOfAttributes with the first request? – matteo Sep 4 '17 at 11:43

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 )
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    setSelectedFeatures should work up to QGIS2.16 but will be/has been deprecated in newer versions. See here. New coders should try this instead: lyr.selectByIds( idsToSel ) – weiji14 Aug 10 '16 at 1:53
  • That's right, thanks! There are upcoming API changes that will make tons of GIS.SE answers obsolete. – Germán Carrillo Aug 10 '16 at 13:32

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.

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