Selecting and manipulating multiple features using the QgsFeatureRequest().setFilterExpression() function in PyQGIS

Question

I have a vector layer with an attribute field called "type". I am trying to delete all features whose value for "type" is not "primary". Here is my little script, which I made thanks to answers from here and here :

from qgis.core import *
with edit(layer):
    request = QgsFeatureRequest().setFilterExpression("\"type\" != 'primary'")
    request.setSubsetOfAttributes([])
    request.setFlags(QgsFeatureRequest.NoGeometry)   
    selection = layer.getFeatures(request)
    layer.deleteFeatures([f.id() for f in selection])

However, when I run it, nothing happens. I've made sure I'm using the correct layer with

>>>layer.id()

Since there are no errors, I am assuming my filter expression is not formatted correctly. Is that likely the case, or is there a problem with my script logic? I'm running Python 3.7.0 on QGIS 3.4.12.

Answer 1

Your filter expression seems working correctly. But calls to deleteFeatures() are only valid for layers in which edits have been enabled by a call to startEditing(). Changes made to features using this method are not committed to the underlying data provider until a commitChanges() call is made. Any uncommitted changes can be discarded by calling rollBack().

layer.startEditing()
layer.deleteFeatures([f.id() for f in selection])
layer.commitChanges()

please refer the documentation Oficial Documentation