PL/Python - Execute return results

Hi,

plan = plpy.prepare("SELECT last_name FROM my_users WHERE first_name =
$1", [ "text" ])
rv = plpy.execute(plan, [ "name" ], 5)
return rv["last_name"]

If the SELECT command does not return any results, how do I
catch/check for this?

if rv == {} ?

or maybe

try:
rv = plpy.execute(plan, [ "name" ], 5)
return rv["last_name"]
except:
...
else:
...

can't seem to get either to work.