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.