[12.0] Purchase triple discount error in runbot

[manuelcalerosolis]

Working with this module we have discovered an error that also occurs in the runbot.

We create a new purchase with the following data.

units: 48
Price: 0.33
discount: 5%
total order: 15,05

These calculations are correct.

We confirm the budget, and receive the products.

The price of the order is still good.

If we make the invoice, we see that the amount has changed.

We are trying to check the calculations to see where the error occurs.

Regards.

[legalsylvain]

Check rounding errors.
See :
48 * .33 * .95 = 15.05....
48 * round(.33 * .95, 2) = 14.88...

[manuelcalerosolis]

If I uninstall the modules "tripe_discount" the calculations are correct.

Something is wrong with these modules.

[manuelcalerosolis]

The only solution is to completely rewrite the method "_compute_price".

I know it is not a good idea, possible changes in this method will not be contemplated, and this is dangerous.

The ideal would be to propose a change so that the price of the product already with discounts is in a single method, and the code is not so coupled.

Do you think anyone would listen to this proposal?

class AccountInvoiceLine(models.Model):
_inherit = "account.invoice.line"

discount2 = fields.Float(
    'Discount 2 (%)',
    digits=dp.get_precision('Discount'),
)
discount3 = fields.Float(
    'Discount 3 (%)',
    digits=dp.get_precision('Discount'),
)

@api.multi
@api.depends('discount2', 'discount3')
def _compute_price(self):
    currency = self.invoice_id and self.invoice_id.currency_id or None
    
    # to avoid coupling these calculations must be passed to a method 
    price = self.price_unit * (1 - (self.discount or 0.0) / 100.0)
    price *= (1 - (self.discount2 or 0.0) / 100.0)
    price *= (1 - (self.discount3 or 0.0) / 100.0)

    taxes = False
    if self.invoice_line_tax_ids:
        taxes = self.invoice_line_tax_ids.compute_all(price, currency, self.quantity, product=self.product_id,
                                                      partner=self.invoice_id.partner_id)
    self.price_subtotal = price_subtotal_signed = taxes['total_excluded'] if taxes else self.quantity * price
    self.price_total = taxes['total_included'] if taxes else self.price_subtotal
    if self.invoice_id.currency_id and self.invoice_id.currency_id != self.invoice_id.company_id.currency_id:
        currency = self.invoice_id.currency_id
        date = self.invoice_id._get_currency_rate_date()
        price_subtotal_signed = currency._convert(price_subtotal_signed, self.invoice_id.company_id.currency_id,
                                                  self.company_id or self.env.user.company_id,
                                                  date or fields.Date.today())
    sign = self.invoice_id.type in ['in_refund', 'out_refund'] and -1 or 1
    self.price_subtotal_signed = price_subtotal_signed * sign`

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