When a function is used as a trigger, the dictionary
   TD contains trigger-related values:
   
TD["event"]
       contains the event as a string:
       INSERT, UPDATE,
       DELETE, or TRUNCATE.
      
TD["when"]
       contains one of BEFORE, AFTER, or
       INSTEAD OF.
      
TD["level"]
       contains ROW or STATEMENT.
      
TD["new"]TD["old"]For a row-level trigger, one or both of these fields contain the respective trigger rows, depending on the trigger event.
TD["name"]contains the trigger name.
TD["table_name"]contains the name of the table on which the trigger occurred.
TD["table_schema"]contains the schema of the table on which the trigger occurred.
TD["relid"]contains the OID of the table on which the trigger occurred.
TD["args"]
       If the CREATE TRIGGER command
       included arguments, they are available in TD["args"][0] to
       TD["args"][.
      n-1]
   If TD["when"] is BEFORE or
   INSTEAD OF and
   TD["level"] is ROW, you can
   return None or "OK" from the
   Python function to indicate the row is unmodified,
   "SKIP" to abort the event, or if TD["event"]
   is INSERT or UPDATE you can return
   "MODIFY" to indicate you've modified the new row.
   Otherwise the return value is ignored.