copy_from() with a list of columns?

PostgreSQL's COPY has a way to specify a subset of the columns:

http://www.postgresql.org/docs/current/static/sql-copy.html

COPY tablename [ ( column [, ...] ) ]
    FROM { 'filename' | STDIN }
   ...

But psycopg's copy_from seems to have no way to specify the columns
you use. In my schema, some columns have a default value (now() or
SERIAL) and I don't want to have to specify a value. How to do it with
psycopg?