[web2py] putting a condition in a form

[mart]

Hi,

does anyone know if I can have some sort of condition in a form (but
BEFORE the form is submitted)?

For example, lets say I have 5 fields in a form where depending on
what a user decides, we would expose 2 extra fields (or not). Anyway
to do something like this? I am open to any suggestion :)

thanks,
Mart :)