Problem adding datefield to invoice odoo12 - invoice

Question

The following Python script does not create a date field:
from datetime import datetime
from odoo import models, fields, api, _
from odoo.exceptions import ValidationError
class DealInvoice(models.Model):
_inherit = 'account.invoice'
x_date_deal = fields.Date(string='Date Deal',
required=True,
readonly=True,
default=(date.today()),
index=True,
states={'draft': [('readonly', False)], 'sent': [('readonly', False)]},
help='Item date deal.')
#api.constrains('x_date_deal')
def _x_date_deal_check(self):
for record in self:
if record.x_date_deal and record.x_date_deal [0] <= str(date.today()):
raise ValidationError(_("Date deal must be before current date."))

Answer 1

The code hereunder gives the error 'date not defined'. In the end i just want avalidation that a date is today or not more than 5 days in the past. Thanks for your help.
from odoo import api, fields, models, _
from odoo.exceptions import ValidationError
from datetime import date
today = date.today
class AccountInvoice(models.Model):
_inherit = 'account.invoice'
_name = 'account.invoice'
x_date_deal = fields.Date(string='Date of Deal', required=True, readonly=False, index=True,
states={'draft': [('readonly', False)], 'sent': [('readonly', False)]},
help='Date of Deal')
#api.constrains('x_date_deal')
def _delivery_date_check(self):
for record in self:
if record.x_date_deal and record.x_date_deal > today:
raise ValidationError(_("Deal Date must be ...."))