Selecting and reselecting ObjectID/OID using arcpy

I was searching for something else when I stumbled upon a solution to another problem I had a while back.

With arcpy you can Describe a layer to find its FIDSet – a string containing the OIDs of all selected features.  I’ve found this list to be useful in scripts so I can go back to previously selected features, or perform analysis based on selected features etc., however there was no way to pass an updated FIDSet back to the layer – it is read only.

Enter the getSelectionSet() and setSelectionSet(), which are methods attached to the Layer class.  Where FIDSet returns a string of OIDs, getSelectionSet returns a python list which can be looped through and modified as required, and then passed back (again as a python list) to update the selection by using setSelectionSet.  I’ve found this to be much faster at selecting features than passing a query with all OIDs to the standard selectLayerByAttribute.

In my test run, looping through 5000 at a time to a total of 55000 records, using FIDSet and SelectLayerByAttribute took close to 2 minutes, and using setSelectionSet took only 24 seconds.

import arcpy
 
FeatureLayer = "MyLayer"
myObjectIDList = [27, 35, 58, 87, 985, 1287] # List of ObjectIDs to select from my FeatureLayer 
 
lyr = arcpy.mapping.Layer(FeatureLayer)
lyr.setSelectionSet("NEW", myObjectIDList) # Select features