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