i have a code of periode

'periode' : fields.many2one('account.period','Periode'),

and default

def default_abcd(self, cr, uid, context={}):

return '12'

_defaults = {

        'abcd' : default_abcd,

          }

and 1 function fields to accomodate (abcd + another fields.float)

and how if i want to abcd return back into '12' when periode change from 2013 to 2014