or better:
db.define_table('test', SQLField('field1'), SQLField('field2'))
db.test.field1.requires=IS_IN_SET(('a','b','c'))
db.test.field1.widget = SQLFORM.widgets.radio.widget
db.test.field2.requires=IS_IN_SET(('a','b','c'),multiple=True)
db.test.field2.widget = SQLFORM.widgets.checkboxes.widget