[web2py] DAL db(query).update(variable=xxx) how?

[Fran]

I want to db(query).update(fieldname=XXX) where fieldname is a
variable, but whatever I try (like eval(fieldname) or `fieldname`) I
get:

SyntaxError: keyword can't be an expression

Must be some trick I'm missing?

Many thanks,
Fran.