Shapefile (vector layer) for particular selected features in PyQGIS

Question

I want to create a new shapefile (vector layer) from selected features. I select one feature then save that feature to a new shapefile.

When I use following code, I got a new shapefile but it's blank (blank attribute table, only field names available in the table). I want row entries based on the feature which is selected.

      filename = 'C:/temp/outputs/select_001.shp'
      selectedLayer = self.dlg.comboBox.currentLayer()
      selectedField = self.dlg.comboBox_2.currentField()
      selectedFeature = self.dlg.comboBox_3.currentText()
      selectedLayer.selectByExpression( "\"FIELD\"='{}' = \"FEAT\"='{}'".format( selectedField, selectedFeature ))
      #selection = layer.getFeatures(request)
      writer = QgsVectorFileWriter.writeAsVectorFormat(selectedLayer, filename, 'utf-8', driverName='ESRI Shapefile', onlySelected=True)
      selected_layer = self.iface.addVectorLayer(filename, '', 'ogr')
      del(writer)

Answer 1

Your expression is wrong. You shouldn't have the strings "FIELD" or "FEAT" in your expression. What you are trying to check is "Field_name" LIKE 'Some string', and inserting your actual selected field name and selected value. Alternatively, you could use something like:

selectedLayer.selectByIds([f.id() for f in selectedLayer.getFeatures() if str(f[selectedField]) == selectedFeature])

Which should work for both text and numeric fields but may be slower on large datasets.

Also QgsVectorFileWriter.writeAsVectorFormat is deprecated for a while now. Use QgsVectorFileWriter.writeAsVectorFormatV2 instead (and after 3.20 this is also deprecated for writeAsVectorFormatV3). Try this (* don't forget to add QgsProject to your imports!):

    filename = 'C:/temp/outputs/select_001.shp'
    selectedLayer = self.dlg.comboBox.currentLayer()
    selectedField = self.dlg.comboBox_2.currentField()
    selectedFeature = self.dlg.comboBox_3.currentText()
    selectedLayer.selectByExpression( '"{}" LIKE \'{}\''.format( selectedField, selectedFeature ))
    # Or:
    #selectedLayer.selectByIds([f.id() for f in selectedLayer.getFeatures() if str(f[selectedField]) == selectedFeature])
    save_options = QgsVectorFileWriter.SaveVectorOptions()
    save_options.driverName = 'ESRI Shapefile'
    save_options.onlySelectedFeatures = True
    err, result = QgsVectorFileWriter.writeAsVectorFormatV2(selectedLayer, filename, QgsProject.instance().transformContext(), save_options)
    selectedLayer.removeSelection()
    if err == 0:
        self.iface.addVectorLayer(filename, '', 'ogr')