Accrue and allocate rounding errors when there are multiple taxes

Closes #11317

modules/account/CHANGELOG

@@ -1,3 +1,4 @@
+* Accrue and allocate rounding errors when there are multiple taxes
 * Use match pattern to find journal
 * Display general ledger information on account form
 

modules/account/tax.py

@@ -3,7 +3,7 @@
 import datetime
 from collections import namedtuple
 from decimal import Decimal
-from itertools import groupby
+from itertools import cycle, groupby
 
 from sql import Literal
 from sql.aggregate import Sum
@@ -1248,8 +1248,23 @@ class TaxableMixin(object):
     def _round_taxes(self, taxes):
         if not self.currency:
             return
+
+        remainder = 0
         for taxline in taxes.values():
-            taxline['amount'] = self.currency.round(taxline['amount'])
+            rounded_amount = self.currency.round(taxline['amount'])
+            remainder += rounded_amount - taxline['amount']
+            taxline['amount'] = rounded_amount
+
+        # We need to compensate the rounding we did
+        remainder = self.currency.round(remainder, opposite=True)
+        if abs(remainder) >= self.currency.rounding:
+            offset_amount = self.currency.rounding.copy_sign(remainder)
+
+            for tax in cycle(taxes.values()):
+                tax['amount'] -= offset_amount
+                remainder -= offset_amount
+                if abs(remainder) < self.currency.rounding:
+                    break
 
     @fields.depends('company', methods=['_get_tax_context', '_round_taxes'])
     def _get_taxes(self):
@@ -1267,6 +1282,7 @@ class TaxableMixin(object):
                 assert all(t.company == self.company for t in line.taxes)
                 l_taxes = Tax.compute(Tax.browse(line.taxes), line.unit_price,
                     line.quantity, line.tax_date or self.tax_date)
+                current_taxes = {}
                 for tax in l_taxes:
                     taxline = self._compute_tax_line(**tax)
                     # Base must always be rounded per line as there will be one
@@ -1278,8 +1294,9 @@ class TaxableMixin(object):
                     else:
                         taxes[taxline]['base'] += taxline['base']
                         taxes[taxline]['amount'] += taxline['amount']
+                    current_taxes[taxline] = taxes[taxline]
                 if tax_rounding == 'line':
-                    self._round_taxes(taxes)
+                    self._round_taxes(current_taxes)
         if tax_rounding == 'document':
             self._round_taxes(taxes)
         return taxes

modules/account/tests/test_module.py

@@ -1153,6 +1153,108 @@ class AccountTestCase(
         self.assertEqual(tax['base'], Decimal('100.00'))
         self.assertEqual(tax['amount'], Decimal('20.02'))
 
+    @with_transaction()
+    def test_taxable_mixin_tax_residual_rounding(self):
+        "Test TaxableMixin for rounding with residual amount"
+        pool = Pool()
+        Account = pool.get('account.account')
+        Tax = pool.get('account.tax')
+        Configuration = pool.get('account.configuration')
+        currency = create_currency('cur')
+
+        company = create_company()
+        with set_company(company):
+            create_chart(company)
+
+            config = Configuration(1)
+            config.tax_rounding = 'line'
+            config.save()
+
+            tax_account, = Account.search([
+                    ('name', '=', 'Main Tax'),
+                    ])
+
+            tax1 = Tax()
+            tax1.name = tax1.description = "Tax 1"
+            tax1.type = 'percentage'
+            tax1.rate = Decimal('.1')
+            tax1.invoice_account = tax_account
+            tax1.credit_note_account = tax_account
+            tax1.save()
+            tax2 = Tax()
+            tax2.name = tax2.description = "Tax 2"
+            tax2.type = 'percentage'
+            tax2.rate = Decimal('.1')
+            tax2.invoice_account = tax_account
+            tax2.credit_note_account = tax_account
+            tax2.save()
+
+            taxable = self.Taxable(
+                currency=currency,
+                taxable_lines=[
+                    ([tax1, tax2], Decimal('1.0417'), 1, None),
+                    ],
+                company=company)
+
+            taxline_1, taxline_2 = taxable._get_taxes()
+            self.assertEqual(taxline_1['base'], Decimal('1.04'))
+            self.assertEqual(taxline_1['amount'], Decimal('.11'))
+            self.assertEqual(taxline_2['base'], Decimal('1.04'))
+            self.assertEqual(taxline_2['amount'], Decimal('.10'))
+
+    @with_transaction()
+    def test_taxable_mixin_tax_residual_rounding_negative_residual(self):
+        "Test TaxableMixin for rounding with negative residual amount"
+        pool = Pool()
+        Account = pool.get('account.account')
+        Tax = pool.get('account.tax')
+        Configuration = pool.get('account.configuration')
+        currency = create_currency('cur')
+
+        company = create_company()
+        with set_company(company):
+            create_chart(company)
+
+            config = Configuration(1)
+            config.tax_rounding = 'line'
+            config.save()
+
+            tax_account, = Account.search([
+                    ('name', '=', 'Main Tax'),
+                    ])
+
+            tax1 = Tax()
+            tax1.name = tax1.description = "Tax 1"
+            tax1.type = 'percentage'
+            tax1.rate = Decimal('.1')
+            tax1.invoice_account = tax_account
+            tax1.credit_note_account = tax_account
+            tax1.save()
+            tax2 = Tax()
+            tax2.name = tax2.description = "Tax 2"
+            tax2.type = 'percentage'
+            tax2.rate = Decimal('.1')
+            tax2.invoice_account = tax_account
+            tax2.credit_note_account = tax_account
+            tax2.save()
+
+            # -2.95 is the unit price of -3.54 with 20% tax included
+            taxable = self.Taxable(
+                currency=currency,
+                taxable_lines=[
+                    ([tax1], Decimal('30.00'), 1, None),
+                    ([tax1, tax2], Decimal('-2.95'), 1, None),
+                    ],
+                company=company)
+
+            taxline_1, taxline_2, taxline_3 = taxable._get_taxes()
+            self.assertEqual(taxline_1['base'], Decimal('30.00'))
+            self.assertEqual(taxline_1['amount'], Decimal('3.00'))
+            self.assertEqual(taxline_2['base'], Decimal('-2.95'))
+            self.assertEqual(taxline_2['amount'], Decimal('-0.29'))
+            self.assertEqual(taxline_3['base'], Decimal('-2.95'))
+            self.assertEqual(taxline_3['amount'], Decimal('-0.30'))
+
     @with_transaction()
     def test_tax_compute_with_children_update_unit_price(self):
         "Test tax compute with children taxes modifying unit_price"