PostgreSQL

45.6. Trigger Functions
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45.6. Trigger Functions

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. [.term]#TD["new"]
[.term]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.

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45.7. Database Access