Validators are classes used to validate input fields (including forms generated from database tables). With the advanced forms derived from SQLFORM, validators create widgets such as drop-down menus and lookups from other tables. Here is an example of using a validator with a FORM:
## Input form field usage
INPUT(_name='a', requires=IS_INT_IN_RANGE(0, 10))
## Validator for a table field
db.define_table('person', Field('name'))
db.person.name.requires = IS_NOT_EMPTY()
