TaxRules: Add internal_name and keep_gross_if_rate_changes (#2422)

Co-authored-by: ser8phin <eva.wolkwitz@gmx.de>

doc/api/resources/taxrules.rst

@@ -16,15 +16,22 @@ Field                                 Type                       Description
 ===================================== ========================== =======================================================
 id                                    integer                    Internal ID of the tax rule
 name                                  multi-lingual string       The tax rules' name
+internal_name                         string                     An optional name that is only used in the backend
 rate                                  decimal (string)           Tax rate in percent
 price_includes_tax                    boolean                    If ``true`` (default), tax is assumed to be included in
                                                                  the specified product price
 eu_reverse_charge                     boolean                    If ``true``, EU reverse charge rules are applied
 home_country                          string                     Merchant country (required for reverse charge), can be
                                                                  ``null`` or empty string
+keep_gross_if_rate_changes            boolean                    If ``true``, changes of the tax rate based on custom
+                                                                 rules keep the gross price constant (default is ``false``)
 ===================================== ========================== =======================================================
 
 
+.. versionchanged:: 4.6
+
+    The ``internal_name`` and ``keep_gross_if_rate_changes`` attributes have been added.
+
 Endpoints
 ---------
 
@@ -56,9 +63,11 @@ Endpoints
           {
             "id": 1,
             "name": {"en": "VAT"},
+            "internal_name": "VAT",
             "rate": "19.00",
             "price_includes_tax": true,
             "eu_reverse_charge": false,
+            "keep_gross_if_rate_changes": false,
             "home_country": "DE"
           }
         ]
@@ -94,9 +103,11 @@ Endpoints
       {
         "id": 1,
         "name": {"en": "VAT"},
+        "internal_name": "VAT",
         "rate": "19.00",
         "price_includes_tax": true,
         "eu_reverse_charge": false,
+        "keep_gross_if_rate_changes": false,
         "home_country": "DE"
       }
 
@@ -140,9 +151,11 @@ Endpoints
       {
         "id": 1,
         "name": {"en": "VAT"},
+        "internal_name": "VAT",
         "rate": "19.00",
         "price_includes_tax": true,
         "eu_reverse_charge": false,
+        "keep_gross_if_rate_changes": false,
         "home_country": "DE"
       }
 
@@ -185,9 +198,11 @@ Endpoints
       {
         "id": 1,
         "name": {"en": "VAT"},
+        "internal_name": "VAT",
         "rate": "20.00",
         "price_includes_tax": true,
         "eu_reverse_charge": false,
+        "keep_gross_if_rate_changes": false,
         "home_country": "DE"
       }
 

src/pretix/api/serializers/event.py

@@ -637,7 +637,7 @@ class SubEventSerializer(I18nAwareModelSerializer):
 class TaxRuleSerializer(CountryFieldMixin, I18nAwareModelSerializer):
     class Meta:
         model = TaxRule
-        fields = ('id', 'name', 'rate', 'price_includes_tax', 'eu_reverse_charge', 'home_country')
+        fields = ('id', 'name', 'rate', 'price_includes_tax', 'eu_reverse_charge', 'home_country', 'internal_name', 'keep_gross_if_rate_changes')
 
 
 class EventSettingsSerializer(SettingsSerializer):

src/pretix/base/migrations/0207_auto_20220119_1427.py

@@ -0,0 +1,23 @@
+# Generated by Django 3.2.4 on 2022-01-19 14:27
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0206_customer_phone'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='taxrule',
+            name='internal_name',
+            field=models.CharField(max_length=190, null=True),
+        ),
+        migrations.AddField(
+            model_name='taxrule',
+            name='keep_gross_if_rate_changes',
+            field=models.BooleanField(default=False),
+        ),
+    ]

src/pretix/base/models/tax.py

@@ -81,6 +81,15 @@ class TaxedPrice:
             name=self.name,
         )
 
+    def __eq__(self, other):
+        return (
+            self.gross == other.gross and
+            self.net == other.net and
+            self.tax == other.tax and
+            self.rate == other.rate and
+            self.name == other.name
+        )
+
 
 TAXED_ZERO = TaxedPrice(
     gross=Decimal('0.00'),
@@ -127,8 +136,13 @@ def cc_to_vat_prefix(country_code):
 
 class TaxRule(LoggedModel):
     event = models.ForeignKey('Event', related_name='tax_rules', on_delete=models.CASCADE)
+    internal_name = models.CharField(
+        verbose_name=_('Internal name'),
+        max_length=190,
+        null=True, blank=True,
+    )
     name = I18nCharField(
-        verbose_name=_('Name'),
+        verbose_name=_('Official name'),
         help_text=_('Should be short, e.g. "VAT"'),
         max_length=190,
     )
@@ -141,6 +155,10 @@ class TaxRule(LoggedModel):
         verbose_name=_("The configured product prices include the tax amount"),
         default=True,
     )
+    keep_gross_if_rate_changes = models.BooleanField(
+        verbose_name=_("Keep gross amount constant if the tax rate changes based on the invoice address"),
+        default=False,
+    )
     eu_reverse_charge = models.BooleanField(
         verbose_name=_("Use EU reverse charge taxation rules"),
         default=False,
@@ -198,6 +216,8 @@ class TaxRule(LoggedModel):
             s = _('plus {rate}% {name}').format(rate=self.rate, name=self.name)
         if self.eu_reverse_charge:
             s += ' ({})'.format(_('reverse charge enabled'))
+        if self.internal_name:
+            return f'{self.internal_name} ({s})'
         return str(s)
 
     @property
@@ -228,13 +248,19 @@ class TaxRule(LoggedModel):
             rate = override_tax_rate
         elif invoice_address:
             adjust_rate = self.tax_rate_for(invoice_address)
-            if (adjust_rate == gross_price_is_tax_rate or force_fixed_gross_price) and base_price_is == 'gross':
+            if (adjust_rate == gross_price_is_tax_rate or force_fixed_gross_price or self.keep_gross_if_rate_changes) and base_price_is == 'gross':
                 rate = adjust_rate
             elif adjust_rate != rate:
-                normal_price = self.tax(base_price, base_price_is, currency, subtract_from_gross=subtract_from_gross)
-                base_price = normal_price.net
-                base_price_is = 'net'
-                subtract_from_gross = Decimal('0.00')
+                if self.keep_gross_if_rate_changes:
+                    normal_price = self.tax(base_price, base_price_is, currency, subtract_from_gross=subtract_from_gross)
+                    base_price = normal_price.gross
+                    base_price_is = 'gross'
+                    subtract_from_gross = Decimal('0.00')
+                else:
+                    normal_price = self.tax(base_price, base_price_is, currency, subtract_from_gross=subtract_from_gross)
+                    base_price = normal_price.net
+                    base_price_is = 'net'
+                    subtract_from_gross = Decimal('0.00')
                 rate = adjust_rate
 
         if rate == Decimal('0.00'):

src/pretix/base/services/cart.py

@@ -426,10 +426,10 @@ class CartManager:
                 if not cp.includes_tax:
                     price = self._get_price(cp.item, cp.variation, cp.voucher, cp.price, cp.subevent,
                                             cp_is_net=True, bundled_sum=bundled_sum)
-                    price = TaxedPrice(net=price.net, gross=price.net, rate=0, tax=0, name='')
+                    price = TaxedPrice(net=price.net, gross=price.net, rate=Decimal('0'), tax=Decimal('0'), name='')
                     pbv = self._get_price(cp.item, cp.variation, None, cp.price, cp.subevent,
                                           cp_is_net=True, bundled_sum=bundled_sum)
-                    pbv = TaxedPrice(net=pbv.net, gross=pbv.net, rate=0, tax=0, name='')
+                    pbv = TaxedPrice(net=pbv.net, gross=pbv.net, rate=Decimal('0'), tax=Decimal('0'), name='')
                 else:
                     price = self._get_price(cp.item, cp.variation, cp.voucher, cp.price, cp.subevent,
                                             bundled_sum=bundled_sum)
@@ -1106,10 +1106,11 @@ def update_tax_rates(event: Event, cart_id: str, invoice_address: InvoiceAddress
         rate = pos.item.tax_rule.tax_rate_for(invoice_address)
 
         if pos.tax_rate != rate:
-            current_net = pos.price - pos.tax_value
-            new_gross = pos.item.tax(current_net, base_price_is='net', invoice_address=invoice_address).gross
-            totaldiff += new_gross - pos.price
-            pos.price = new_gross
+            if not pos.item.tax_rule.keep_gross_if_rate_changes:
+                current_net = pos.price - pos.tax_value
+                new_gross = pos.item.tax(current_net, base_price_is='net', invoice_address=invoice_address).gross
+                totaldiff += new_gross - pos.price
+                pos.price = new_gross
             pos.includes_tax = rate != Decimal('0.00')
             pos.override_tax_rate = rate
             pos.save(update_fields=['price', 'includes_tax', 'override_tax_rate'])

src/pretix/base/services/orders.py

@@ -700,7 +700,7 @@ def _check_positions(event: Event, now_dt: datetime, positions: List[CartPositio
                                 invoice_address=address, force_custom_price=True, max_discount=max_discount)
                 changed_prices[cp.pk] = bprice
             else:
-                bundled_sum = 0
+                bundled_sum = Decimal('0.00')
                 if not cp.addon_to_id:
                     for bundledp in cp.addons.all():
                         if bundledp.is_bundled:

src/pretix/control/forms/event.py

@@ -1249,7 +1249,7 @@ TaxRuleLineFormSet = formset_factory(
 class TaxRuleForm(I18nModelForm):
     class Meta:
         model = TaxRule
-        fields = ['name', 'rate', 'price_includes_tax', 'eu_reverse_charge', 'home_country']
+        fields = ['name', 'rate', 'price_includes_tax', 'eu_reverse_charge', 'home_country', 'internal_name', 'keep_gross_if_rate_changes']
 
 
 class WidgetCodeForm(forms.Form):

src/pretix/control/forms/orders.py

@@ -452,7 +452,7 @@ class OrderPositionChangeForm(forms.Form):
 
     @staticmethod
     def taxrule_label_from_instance(obj):
-        return f"{obj.name} ({obj.rate} %)"
+        return f"{obj.internal_name or obj.name} ({obj.rate} %)"
 
     def __init__(self, *args, **kwargs):
         instance = kwargs.pop('instance')

src/pretix/control/templates/pretixcontrol/event/tax_edit.html

@@ -24,6 +24,7 @@
                     <fieldset>
                         <legend>{% trans "General" %}</legend>
                         {% bootstrap_field form.name layout="control" %}
+                        {% bootstrap_field form.internal_name layout="control" %}
                         {% bootstrap_field form.rate addon_after="%" layout="control" %}
                         {% bootstrap_field form.price_includes_tax layout="control" %}
                     </fieldset>
@@ -39,6 +40,7 @@
                         </div>
                         {% bootstrap_field form.eu_reverse_charge layout="control" %}
                         {% bootstrap_field form.home_country layout="control" %}
+                        {% bootstrap_field form.keep_gross_if_rate_changes layout="control" %}
                         <h3>{% trans "Custom taxation rules" %}</h3>
                         <div class="alert alert-warning">
                             {% blocktrans trimmed %}

src/pretix/control/templates/pretixcontrol/event/tax_index.html

@@ -33,7 +33,7 @@
                     <tr>
                         <td>
                             <strong><a href="{% url "control:event.settings.tax.edit" organizer=request.event.organizer.slug event=request.event.slug rule=tr.id %}">
-                                {{ tr.name }}
+                                {{ tr.internal_name|default:tr.name }}
                             </a></strong>
                         </td>
                         <td>

src/pretix/control/templates/pretixcontrol/order/change.html

@@ -145,7 +145,12 @@
                                 <strong>{% trans "Tax rule" %}</strong>
                             </div>
                             <div class="col-sm-5">
-                                {{ position.tax_rule.name }} ({{ position.tax_rule.rate }} %)
+                                {% if position.tax_rule.internal_name %}
+                                    {{ position.tax_rule.internal_name }}
+                                {% else %}
+                                    {{ position.tax_rule.name }}
+                                {% endif %}
+                                ({{ position.tax_rule.rate }} %)
                             </div>
                             <div class="col-sm-4 field-container">
                                 {% bootstrap_field position.form.tax_rule layout='inline' %}

src/tests/api/test_taxrules.py

@@ -27,6 +27,8 @@ from django_scopes import scopes_disabled
 from pretix.base.models import TaxRule
 
 TEST_TAXRULE_RES = {
+    'internal_name': None,
+    'keep_gross_if_rate_changes': False,
     'name': {'en': 'VAT'},
     'rate': '19.00',
     'price_includes_tax': True,

src/tests/base/test_pricing.py

@@ -26,6 +26,7 @@ import pytest
 from django.utils.timezone import now
 from django_countries.fields import Country
 
+from pretix.base.decimal import round_decimal
 from pretix.base.models import Event, InvoiceAddress, Organizer
 from pretix.base.models.items import SubEventItem, SubEventItemVariation
 from pretix.base.services.pricing import get_price
@@ -379,3 +380,77 @@ def test_country_specific_rule_gross_based(item):
         country=Country('BE')
     )
     assert get_price(item, invoice_address=ia).gross == Decimal('168.06')
+
+
+@pytest.mark.django_db
+def test_country_specific_rule_net_based_but_keep_gross_if_rate_changes(item):
+    item.default_price = Decimal('100.00')
+    item.tax_rule = item.event.tax_rules.create(
+        rate=Decimal('19.00'), price_includes_tax=False, keep_gross_if_rate_changes=True,
+        custom_rules=json.dumps([
+            {'country': 'BE', 'address_type': '', 'action': 'vat', 'rate': '100.00'}
+        ])
+    )
+    ia = InvoiceAddress(
+        is_business=True, vat_id="EU1234", vat_id_validated=True,
+        country=Country('BE')
+    )
+    p = get_price(item, invoice_address=ia)
+    assert p.gross == Decimal('119.00')
+    assert p.rate == Decimal('100.00')
+    assert p.tax == Decimal('59.50')
+
+
+@pytest.mark.django_db
+def test_country_specific_rule_net_based_subtract_bundled(item):
+    item.default_price = Decimal('100.00')
+    item.tax_rule = item.event.tax_rules.create(
+        rate=Decimal('19.00'), price_includes_tax=False,
+        custom_rules=json.dumps([
+            {'country': 'BE', 'address_type': '', 'action': 'vat', 'rate': '100.00'}
+        ])
+    )
+    ia = InvoiceAddress(
+        is_business=True, vat_id="EU1234", vat_id_validated=True,
+        country=Country('BE')
+    )
+    assert get_price(item, invoice_address=ia, bundled_sum=Decimal('20.00')).gross == (
+        round_decimal((Decimal('119.00') - Decimal('20.00')) / Decimal('1.19')) * Decimal('2')
+    )
+
+
+@pytest.mark.django_db
+def test_country_specific_rule_gross_based_subtract_bundled(item):
+    item.default_price = Decimal('100.00')
+    item.tax_rule = item.event.tax_rules.create(
+        rate=Decimal('19.00'), price_includes_tax=True,
+        custom_rules=json.dumps([
+            {'country': 'BE', 'address_type': '', 'action': 'vat', 'rate': '100.00'}
+        ])
+    )
+    ia = InvoiceAddress(
+        is_business=True, vat_id="EU1234", vat_id_validated=True,
+        country=Country('BE')
+    )
+    assert get_price(item, invoice_address=ia, bundled_sum=Decimal('20.00')).gross == (
+        round_decimal((Decimal('100.00') - Decimal('20.00')) / Decimal('1.19')) * Decimal('2')
+    )
+
+
+@pytest.mark.django_db
+def test_country_specific_rule_net_based_but_keep_gross_if_rate_changes_subtract_bundled(item):
+    item.default_price = Decimal('100.00')
+    item.tax_rule = item.event.tax_rules.create(
+        rate=Decimal('19.00'), price_includes_tax=False, keep_gross_if_rate_changes=True,
+        custom_rules=json.dumps([
+            {'country': 'BE', 'address_type': '', 'action': 'vat', 'rate': '100.00'}
+        ])
+    )
+    ia = InvoiceAddress(
+        is_business=True, vat_id="EU1234", vat_id_validated=True,
+        country=Country('BE')
+    )
+    p = get_price(item, invoice_address=ia, bundled_sum=Decimal('20.00'))
+    assert p.gross == Decimal('99.00')
+    assert p.rate == Decimal('100.00')
+    assert p.tax == Decimal('49.50')

src/tests/base/test_taxrules.py

@@ -28,6 +28,7 @@ from django_countries.fields import Country
 from django_scopes import scope
 
 from pretix.base.models import Event, InvoiceAddress, Organizer, TaxRule
+from pretix.base.models.tax import TaxedPrice
 
 
 @pytest.fixture
@@ -88,6 +89,13 @@ def test_reverse_charge_no_country(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -103,6 +111,13 @@ def test_reverse_charge_individual_same_country(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -118,6 +133,13 @@ def test_reverse_charge_individual_eu(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -133,6 +155,13 @@ def test_reverse_charge_individual_3rdc(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -148,6 +177,13 @@ def test_reverse_charge_business_same_country(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -163,6 +199,13 @@ def test_reverse_charge_business_eu(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -178,6 +221,13 @@ def test_reverse_charge_business_3rdc(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -195,6 +245,13 @@ def test_reverse_charge_valid_vat_id_business_same_country(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -212,6 +269,13 @@ def test_reverse_charge_valid_vat_id_business_eu(event):
     assert tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -229,6 +293,13 @@ def test_reverse_charge_valid_vat_id_business_3rdc(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -246,6 +317,13 @@ def test_reverse_charge_disabled(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -266,6 +344,13 @@ def test_custom_rules_override(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -287,6 +372,13 @@ def test_custom_rules_in_order(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -305,6 +397,13 @@ def test_custom_rules_any_country(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -329,6 +428,13 @@ def test_custom_rules_eu_country(event):
     )
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -347,6 +453,14 @@ def test_custom_rules_specific_country(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
+
     ia = InvoiceAddress(
         is_business=True,
         country=Country('DE')
@@ -354,6 +468,13 @@ def test_custom_rules_specific_country(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -372,6 +493,14 @@ def test_custom_rules_individual(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
+
     ia = InvoiceAddress(
         is_business=True,
         country=Country('DE')
@@ -379,6 +508,13 @@ def test_custom_rules_individual(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -397,6 +533,14 @@ def test_custom_rules_business(event):
     assert not tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
+
     ia = InvoiceAddress(
         is_business=False,
         country=Country('DE')
@@ -404,6 +548,13 @@ def test_custom_rules_business(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -422,6 +573,14 @@ def test_custom_rules_vat_id(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
+
     ia = InvoiceAddress(
         is_business=True,
         country=Country('DE'),
@@ -431,6 +590,13 @@ def test_custom_rules_vat_id(event):
     assert tr.is_reverse_charge(ia)
     assert not tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('0.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('100.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('0.00'),
+        rate=Decimal('0.00'),
+        name='',
+    )
 
 
 @pytest.mark.django_db
@@ -449,6 +615,13 @@ def test_custom_rules_country_rate(event):
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
     assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
     ia = InvoiceAddress(
         is_business=True,
         country=Country('DE'),
@@ -458,3 +631,79 @@ def test_custom_rules_country_rate(event):
     assert tr.tax_rate_for(ia) == Decimal('100.00')
     assert not tr.is_reverse_charge(ia)
     assert tr._tax_applicable(ia)
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('200.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('100.00'),
+        rate=Decimal('100.00'),
+        name='',
+    )
+
+
+@pytest.mark.django_db
+def test_custom_rules_country_rate_keep_gross_if_rate_changes(event):
+    tr = TaxRule(
+        event=event,
+        rate=Decimal('10.00'), price_includes_tax=False,
+        keep_gross_if_rate_changes=True,
+        custom_rules=json.dumps([
+            {'country': 'EU', 'address_type': 'business_vat_id', 'action': 'vat', 'rate': '100.00'},
+        ])
+    )
+    ia = InvoiceAddress(
+        is_business=True,
+        country=Country('DE')
+    )
+    assert not tr.is_reverse_charge(ia)
+    assert tr._tax_applicable(ia)
+    assert tr.tax_rate_for(ia) == Decimal('10.00')
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('100.00'),
+        tax=Decimal('10.00'),
+        rate=Decimal('10.00'),
+        name='',
+    )
+    ia = InvoiceAddress(
+        is_business=True,
+        country=Country('DE'),
+        vat_id='DE1234',
+        vat_id_validated=True
+    )
+    assert tr.tax_rate_for(ia) == Decimal('100.00')
+    assert not tr.is_reverse_charge(ia)
+    assert tr._tax_applicable(ia)
+    assert tr.tax(Decimal('100.00'), invoice_address=ia) == TaxedPrice(
+        gross=Decimal('110.00'),
+        net=Decimal('55.00'),
+        tax=Decimal('55.00'),
+        rate=Decimal('100.00'),
+        name='',
+    )
+
+
+@pytest.mark.django_db
+def test_custom_rules_country_rate_subtract_from_gross(event):
+    tr = TaxRule(
+        event=event,
+        rate=Decimal('10.00'), price_includes_tax=False,
+        custom_rules=json.dumps([
+            {'country': 'EU', 'address_type': 'business_vat_id', 'action': 'vat', 'rate': '100.00'},
+        ])
+    )
+    ia = InvoiceAddress(
+        is_business=True,
+        country=Country('DE'),
+        vat_id='DE1234',
+        vat_id_validated=True
+    )
+    assert tr.tax_rate_for(ia) == Decimal('100.00')
+    assert not tr.is_reverse_charge(ia)
+    assert tr._tax_applicable(ia)
+    assert tr.tax(Decimal('100.00'), invoice_address=ia, subtract_from_gross=Decimal('20.00')) == TaxedPrice(
+        gross=Decimal('163.64'),  # ((100 * 1.1) - 20) / (1 + 10%) * (1 + 100%)
+        net=Decimal('81.82'),
+        tax=Decimal('81.82'),
+        rate=Decimal('100.00'),
+        name='',
+    )

src/tests/presale/test_cart.py

@@ -3518,6 +3518,39 @@ class CartBundleTest(CartTestMixin, TestCase):
         assert a.tax_rate == Decimal('0.00')
         assert not a.includes_tax
 
+    @classscope(attr='orga')
+    def test_reverse_charge_bundled_add_keep_gross_price(self):
+        ia = InvoiceAddress.objects.create(
+            is_business=True, vat_id='ATU1234567', vat_id_validated=True,
+            country=Country('AT')
+        )
+        tr19 = self.event.tax_rules.create(name='VAT', rate=Decimal('19.00'))
+        tr7 = self.event.tax_rules.create(name='VAT', rate=Decimal('7.00'), eu_reverse_charge=True, home_country=Country('DE'),
+                                          keep_gross_if_rate_changes=True)
+        self.ticket.tax_rule = tr19
+        self.ticket.save()
+        self.trans.tax_rule = tr7
+        self.trans.save()
+
+        self.cm.invoice_address = ia
+        self.cm.add_new_items([
+            {
+                'item': self.ticket.pk,
+                'variation': None,
+                'count': 1
+            }
+        ])
+        self.cm.commit()
+
+        cp = CartPosition.objects.filter(addon_to__isnull=True).get()
+        a = CartPosition.objects.filter(addon_to__isnull=False).get()
+        assert cp.price == Decimal('21.50')
+        assert cp.tax_rate == Decimal('19.00')
+        assert cp.includes_tax
+        assert a.price == Decimal('1.50')
+        assert a.tax_rate == Decimal('0.00')
+        assert not a.includes_tax
+
     @classscope(attr='orga')
     def test_reverse_charge_bundled_add(self):
         ia = InvoiceAddress.objects.create(

src/tests/presale/test_checkout.py

@@ -351,6 +351,42 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
             ia = InvoiceAddress.objects.get(pk=self.client.session['carts'][self.session_key].get('invoice_address'))
         assert not ia.vat_id_validated
 
+    def test_reverse_charge_keep_gross(self):
+        self.tr19.eu_reverse_charge = True
+        self.tr19.keep_gross_if_rate_changes = True
+        self.tr19.home_country = Country('DE')
+        self.tr19.save()
+        self.event.settings.invoice_address_vatid = True
+
+        with scopes_disabled():
+            cr1 = CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=23, expires=now() + timedelta(minutes=10)
+            )
+
+        with mock.patch('vat_moss.id.validate') as mock_validate:
+            mock_validate.return_value = ('AT', 'AT123456', 'Foo')
+            self.client.post('/%s/%s/checkout/questions/' % (self.orga.slug, self.event.slug), {
+                'is_business': 'business',
+                'company': 'Foo',
+                'name': 'Bar',
+                'street': 'Baz',
+                'zipcode': '12345',
+                'city': 'Here',
+                'country': 'AT',
+                'vat_id': 'AT123456',
+                'email': 'admin@localhost'
+            }, follow=True)
+
+        cr1.refresh_from_db()
+        assert cr1.price == Decimal('23.00')
+        assert cr1.tax_rate == Decimal('0.00')
+        assert cr1.tax_value == Decimal('0.00')
+
+        with scopes_disabled():
+            ia = InvoiceAddress.objects.get(pk=self.client.session['carts'][self.session_key].get('invoice_address'))
+        assert ia.vat_id_validated
+
     def test_custom_tax_rules(self):
         self.tr19.custom_rules = json.dumps([
             {'country': 'AT', 'address_type': 'business_vat_id', 'action': 'reverse'},
@@ -585,6 +621,72 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
             cr = CartPosition.objects.get(cart_id=self.session_key)
             assert cr.price == Decimal('21.26')
 
+    def test_free_price_net_price_reverse_charge_keep_gross(self):
+        # This is an end-to-end test of a very confusing case in which the event is set to
+        # "show net prices" but the tax rate is set to "keep gross if rate changes" in
+        # combination of free prices.
+        # This means the user will be greeted with a display price of "23 EUR + VAT". If they
+        # then adjust the price to pay more, e.g. "40 EUR", it will be interpreted as a net
+        # value (since the event is set to shown net values). The cart position is therefore
+        # created with a gross price of 47.60 EUR. Then, the user enters their invoice address, which
+        # triggers reverse charge. The tax is now removed, but since the tax rule is set to
+        # keep the gross price the same, the user will still need to pay 47.60 EUR (incl 0% VAT),
+        # instead of the 40 EUR the maybe wanted in the first place.
+        # While confusing, this behaviour is technically correct and the correct answer to anyone
+        # complaining about this is "do not turn display_net_prices and keep_gross_if_rate_changes
+        # on at the same time" (display_net_prices only makes sense if you're targeting a B2B
+        # audience in which case keep_gross_if_rate_changes is useless or even harmful).
+        self.event.settings.display_net_prices = True
+        self.ticket.free_price = True
+        self.ticket.save()
+        self.tr19.eu_reverse_charge = True
+        self.tr19.keep_gross_if_rate_changes = True
+        self.tr19.price_includes_tax = False
+        self.tr19.home_country = Country('DE')
+        self.tr19.save()
+        self.event.settings.invoice_address_vatid = True
+
+        response = self.client.post('/%s/%s/cart/add' % (self.orga.slug, self.event.slug), {
+            'item_%d' % self.ticket.id: '1',
+            'price_%d' % self.ticket.id: '40.00',
+        }, follow=True)
+        self.assertRedirects(response, '/%s/%s/?require_cookie=true' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+
+        with scopes_disabled():
+            cr1 = CartPosition.objects.get()
+            assert cr1.price == Decimal('47.60')
+            assert cr1.tax_rate == Decimal('19.00')
+
+        with mock.patch('vat_moss.id.validate') as mock_validate:
+            mock_validate.return_value = ('AT', 'AT123456', 'Foo')
+            self.client.post('/%s/%s/checkout/questions/' % (self.orga.slug, self.event.slug), {
+                'is_business': 'business',
+                'company': 'Foo',
+                'name': 'Bar',
+                'street': 'Baz',
+                'zipcode': '12345',
+                'city': 'Here',
+                'country': 'AT',
+                'vat_id': 'AT123456',
+                'email': 'admin@localhost'
+            }, follow=True)
+
+        cr1.refresh_from_db()
+        assert cr1.price == Decimal('47.60')
+        assert cr1.tax_rate == Decimal('0.00')
+
+        self._set_session('payment', 'banktransfer')
+
+        response = self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.content.decode(), "lxml")
+        self.assertEqual(len(doc.select(".thank-you")), 1)
+        with scopes_disabled():
+            op = OrderPosition.objects.get()
+            self.assertEqual(op.price, Decimal('47.60'))
+            self.assertEqual(op.tax_value, Decimal('0.00'))
+            self.assertEqual(op.tax_rate, Decimal('0.00'))
+
     def test_question_file_upload(self):
         with scopes_disabled():
             q1 = Question.objects.create(
@@ -1876,6 +1978,46 @@ class CheckoutTestCase(BaseCheckoutTestCase, TestCase):
             cr1 = CartPosition.objects.get(id=cr1.id)
             self.assertEqual(cr1.price, round_decimal(Decimal('24.00') / Decimal('1.19')))
 
+    def test_confirm_price_changed_reverse_charge_keep_gross(self):
+        self._enable_reverse_charge()
+        self.tr19.keep_gross_if_rate_changes = True
+        self.tr19.save()
+        self.ticket.default_price = 24
+        self.ticket.save()
+        with scopes_disabled():
+            cr1 = CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=23, expires=now() - timedelta(minutes=10)
+            )
+        self._set_session('payment', 'banktransfer')
+
+        response = self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.content.decode(), "lxml")
+        self.assertEqual(len(doc.select(".alert-danger")), 1)
+        with scopes_disabled():
+            cr1 = CartPosition.objects.get(id=cr1.id)
+        self.assertEqual(cr1.price, Decimal('24.00'))
+
+    def test_confirm_price_not_changed_reverse_charge_keep_gross(self):
+        self._enable_reverse_charge()
+        self.tr19.keep_gross_if_rate_changes = True
+        self.tr19.save()
+        with scopes_disabled():
+            CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=23, expires=now() - timedelta(minutes=10)
+            )
+        self._set_session('payment', 'banktransfer')
+
+        response = self.client.post('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.content.decode(), "lxml")
+        self.assertEqual(len(doc.select(".thank-you")), 1)
+        with scopes_disabled():
+            op = OrderPosition.objects.get()
+            self.assertEqual(op.price, Decimal('23.00'))
+            self.assertEqual(op.tax_value, Decimal('0.00'))
+            self.assertEqual(op.tax_rate, Decimal('0.00'))
+
     def test_confirm_price_changed(self):
         self.ticket.default_price = 24
         self.ticket.save()