The valid return value should be MODIFY instead of MODIFIED.

The error message said so :-)

In 25.3. Using PL/Python

        If the trigger "when" is BEFORE, you may return None or "OK"
        from the Python function to indicate the tuple is unmodified, "SKIP"
        to abort the event, or "MODIFIED" to indicate you've modified the tuple.

should read
        If the trigger "when" is BEFORE, you may return None or "OK"
        from the Python function to indicate the tuple is unmodified, "SKIP"
        to abort the event, or "MODIFY" to indicate you've modified the tuple.

elein

doc/src/sgml/plpython.sgml

-<!-- $Header: /cvsroot/pgsql/doc/src/sgml/plpython.sgml,v 1.13 2002/09/22 18:47:24 tgl Exp $ -->
+<!-- $Header: /cvsroot/pgsql/doc/src/sgml/plpython.sgml,v 1.14 2002/09/23 01:51:02 momjian Exp $ -->
 
 <chapter id="plpython">
  <title>PL/Python - Python Procedural Language</title>
@@ -105,7 +105,7 @@ def __plpython_procedure_myfunc_23456():
    If the <literal>TD["when"]</literal> is <literal>BEFORE</>, you may
    return <literal>None</literal> or <literal>"OK"</literal> from the
    Python function to indicate the row is unmodified,
-   <literal>"SKIP"</> to abort the event, or <literal>"MODIFIED"</> to
+   <literal>"SKIP"</> to abort the event, or <literal>"MODIFY"</> to
    indicate you've modified the row.
   </para>
  </sect1>