The variable columnNames is not available in Jython, whereas it is available in GREL. It is useful to iterate over all cells in a row, for instance with this expression:
forEach(row.columnNames,cn,cells[cn].value)
(see https://groups.google.com/g/openrefine/c/gV0bLUz7p7o)

Proposed solution

We could make this variable available in Jython too.
It would also be good to document which variable names (and fields) are available in Jython, if this is really meant to differ from what is available in GREL.

Alternatives considered

Make sure all variables and fields offered in GREL are also available in Jython and Clojure.