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When a function is used in a trigger, the dictionary TD contains trigger-related values. The trigger rows are in TD["new"] and/or TD["old"] depending on the trigger event. TD["event"] contains the event as a string (INSERT, UPDATE, DELETE, or UNKNOWN). TD["when"] contains one of BEFORE, AFTER, and UNKNOWN. TD["level"] contains one of ROW, STATEMENT, and UNKNOWN. TD["name"] contains the trigger name, and TD["relid"] contains the OID of the table on which the trigger occurred. If the trigger was called with arguments they are available in TD["args"][0] to TD["args"][(n-1)].
If TD["when"] is BEFORE, you may return None or "OK" from the Python function to indicate the row is unmodified, "SKIP" to abort the event, or "MODIFY" to indicate you've modified the row.