select by attribute - Deleting selected features using PyQGIS?

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, 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 here

res = Ilayer.dataProvider().deleteFeatures()

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

Answer

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()