For example:

@api.model
def _default_date(self):
  inv_type = self._context.get('type', 'out_invoice')
  if inv_type == 'out_invoice':
     current_date = time.strftime(DEFAULT_SERVER_DATE_FORMAT)
     return current_date

date_invoice = fields.Date(string='Invoice Date',readonly=True, states={'draft': [('readonly', False)]}, index=True, help="Keep empty to use the current date", copy=False, default=_default_date)