I would like to give a default value according to the code of each input. for example if the code is JMOIS give the value 31 in the following code; 

class InputLine(BrowsableObject):
"""a class that will be used into the python code, mainly for usability purposes"""
def sum(self, code, from_date, to_date=None):
if to_date is None:
to_date = fields.Date.today()
self.env.cr.execute("""
SELECT sum(amount) as sum
FROM hr_payslip as hp, hr_payslip_input as pi
WHERE hp.employee_id = %s AND hp.state = 'done'
AND hp.date_from >= %s AND hp.date_to <= %s AND hp.id = pi.payslip_id AND pi.code = %s""",
(self.employee_id, from_date, to_date, code))
return self.env.cr.fetchone()[0] or 0.0