Using get_value_store()

[r.sander (Robert Sander)]

@thl-cmk

    if not persistent_values.get('previous_value'):   # check if this the first run
        persistent_values['previous_value'] = current_value  # save value for next run
        previous_value = None
    else:
        previous_value = persistent_values['previous_value']   # load previos value from store
        persistent_values['previous_value'] = current_value     # svae current value for next run

I would write this block as

previous_value = persistent_values.get('previous_value')
persistent_values['previous_value'] = current_value

There is no if needed as get() returns None when the key is not in the dict. And the second assignment is in both branches of the if.