Writing Python triggers and the TD dictionary value

I can't figure out the plpython syntax to use the TD dictionary values. For example, say I have a column "foo" in table X that has a python trigger defined for it.
The documentation says that TD["old"] will have the original value of the row and TD["new"] will have the new value of the row. I want to get column foo out of TD["old"] and TD["new"]. But everything I try gives me an error:

TD["old"][foo]
TD["old"].foo
TD["old"](foo)

The Perl documentation shows the syntax for this, but the Python version doesn't. Thanks much for any information on the syntax for this!
Please email to curt(dot)schwaderer(at)mchsi(dot)com

Curt