Fixed #132 -- Reverse payment fee calculation

src/pretix/base/payment.py

@@ -25,6 +25,9 @@ class BasePaymentProvider:
     def __init__(self, event: Event):
         self.event = event
         self.settings = SettingsSandbox('payment', self.identifier, event)
+        # Default values
+        if self.settings.get('_fee_reverse_calc') is None:
+            self.settings.set('_fee_reverse_calc', True)
 
     def __str__(self):
         return self.identifier
@@ -48,7 +51,11 @@ class BasePaymentProvider:
         """
         fee_abs = self.settings.get('_fee_abs', as_type=Decimal, default=0)
         fee_percent = self.settings.get('_fee_percent', as_type=Decimal, default=0)
-        return round_decimal(price * fee_percent / 100) + fee_abs
+        fee_reverse_calc = self.settings.get('_fee_reverse_calc', as_type=bool, default=True)
+        if fee_reverse_calc:
+            return round_decimal((price + fee_abs) * (1 / (1 - fee_percent / 100)) - price)
+        else:
+            return round_decimal(price * fee_percent / 100) + fee_abs
 
     @property
     def verbose_name(self) -> str:
@@ -118,6 +125,14 @@ class BasePaymentProvider:
                  help_text=_('Percentage'),
                  required=False
              )),
+            ('_fee_reverse_calc',
+             forms.BooleanField(
+                 label=_('Calculate the fee from the total value including the fee.'),
+                 help_text=_('We recommend you to enable this if you want your users to pay the payment fees of your '
+                             'payment provider. <a href="/control/help/payment/fee_reverse" target="_blank">Click here '
+                             'for detailled information on what this does.</a>'),
+                 required=False
+             )),
         ])
 
     def settings_content_render(self, request: HttpRequest) -> str:

src/pretix/control/templates/pretixcontrol/help/base.html

@@ -0,0 +1,7 @@
+{% extends "pretixcontrol/base.html" %}
+{% load i18n %}
+{% block content %}
+    <h1>{% trans "Help center" %}</h1>
+    {% block inner %}
+    {% endblock %}
+{% endblock %}

src/pretix/control/templates/pretixcontrol/help/payment/fee_reverse.html

@@ -0,0 +1,68 @@
+{% extends "pretixcontrol/help/base.html" %}
+{% block title %}Payment fee calculation{% endblock %}
+{% block inner %}
+    <h2>Payment fee calculation</h2>
+    <p>
+        If you configure a fee for a payment method, there are two possible ways for us to calculate this. Let's
+        assume that your payment provider, e.g. PayPal, charges you 5 % fees and you want to charge your users the
+        same 5 %, such that for a ticket with a list price of 100 € you will get your full 100 €.
+    </p>
+    <ul>
+        <li>
+            <strong>Method A: Calculate the fee from the subtotal and add it to the bill.</strong> For a ticket price of
+            100 €, this will lead to the following calculation:
+            <table class="table" style="width: auto;">
+                <tr>
+                    <td>Ticket price</td>
+                    <td class="text-right">100.00 €</td>
+                </tr>
+                <tr>
+                    <td>pretix calculates the fee as 5% of 100 €</td>
+                    <td class="text-right">+ 5.00 €</td>
+                </tr>
+                <tr>
+                    <td>Subtotal that will be paid by the customer</td>
+                    <td class="text-right">105.00 €</td>
+                </tr>
+                <tr>
+                    <td>PayPal calculates its fee as 5% of 105 €</td>
+                    <td class="text-right">- 5.25 €</td>
+                </tr>
+                <tr>
+                    <td>End total that is on your bank account</td>
+                    <td class="text-right"><strong>99.75 €</strong></td>
+                </tr>
+            </table>
+        </li>
+        <li>
+            <strong>Method B (default): Calculate the fee from the total value including the fee.</strong> For a ticket
+            price of 100 €, this will lead to the following calculation:
+            <table class="table" style="width: auto;">
+                <tr>
+                    <td>Ticket price</td>
+                    <td class="text-right">100.00 €</td>
+                </tr>
+                <tr>
+                    <td>pretix calculates the fee as 100/(100 - 5)% of 100 €</td>
+                    <td class="text-right">+ 5.26 €</td>
+                </tr>
+                <tr>
+                    <td>Subtotal that will be paid by the customer</td>
+                    <td class="text-right">105.26 €</td>
+                </tr>
+                <tr>
+                    <td>PayPal calculates its fee as 5% of 105.26 €</td>
+                    <td class="text-right">- 5.26 €</td>
+                </tr>
+                <tr>
+                    <td>End total that is on your bank account</td>
+                    <td class="text-right"><strong>100.00 €</strong></td>
+                </tr>
+            </table>
+            <div class="alert-warning alert">
+                Due to the various rounding steps performed by us and by the payment provider, the end total on
+                your bank account might stil vary by one cent.
+            </div>
+        </li>
+    </ul>
+{% endblock %}

src/pretix/control/urls.py

@@ -1,8 +1,8 @@
 from django.conf.urls import include, url
 
 from pretix.control.views import (
-    attendees, auth, dashboards, event, item, main, orders, organizer, user,
-    vouchers,
+    attendees, auth, dashboards, event, help, item, main, orders, organizer,
+    user, vouchers,
 )
 
 urlpatterns = [
@@ -78,4 +78,5 @@ urlpatterns = [
         url(r'^orders/$', orders.OrderList.as_view(), name='event.orders'),
         url(r'^attendees/$', attendees.AttendeeList.as_view(), name='event.attendees'),
     ])),
+    url(r'^help/(?P<topic>[^.]+)$', help.HelpView.as_view(), name='help'),
 ]

src/pretix/control/views/help.py

@@ -0,0 +1,23 @@
+from django import template
+from django.http import Http404
+from django.shortcuts import render
+from django.views.generic import View
+
+from pretix.base.models import Organizer
+
+
+class HelpView(View):
+    model = Organizer
+    context_object_name = 'organizers'
+    template_name = 'pretixcontrol/organizers/index.html'
+    paginate_by = 30
+
+    def get(self, request, *args, **kwargs):
+        try:
+            locale = request.LANGUAGE_CODE
+            return render(request, 'pretixcontrol/help/%s.%s.html' % (kwargs.get('topic'), locale), {})
+        except template.TemplateDoesNotExist:
+            try:
+                return render(request, 'pretixcontrol/help/%s.html' % kwargs.get('topic'), {})
+            except template.TemplateDoesNotExist:
+                raise Http404('')

src/tests/base/test_payment.py

@@ -0,0 +1,54 @@
+import time
+from decimal import Decimal
+
+import pytest
+from django.utils.timezone import now
+from tests.testdummy.payment import DummyPaymentProvider
+
+from pretix.base.models import Event, EventLock, Organizer
+from pretix.base.services import locking
+
+
+@pytest.fixture
+def event():
+    o = Organizer.objects.create(name='Dummy', slug='dummy')
+    event = Event.objects.create(
+        organizer=o, name='Dummy', slug='dummy',
+        date_from=now()
+    )
+    return event
+
+
+@pytest.mark.django_db
+def test_payment_fee_forward(event):
+    prov = DummyPaymentProvider(event)
+    prov.settings.set('_fee_abs', Decimal('0.30'))
+    prov.settings.set('_fee_percent', Decimal('5.00'))
+    prov.settings.set('_fee_reverse_calc', False)
+    assert prov.calculate_fee(Decimal('100.00')) == Decimal('5.30')
+
+
+@pytest.mark.django_db
+def test_payment_fee_reverse_percent(event):
+    prov = DummyPaymentProvider(event)
+    prov.settings.set('_fee_abs', Decimal('0.00'))
+    prov.settings.set('_fee_percent', Decimal('5.00'))
+    prov.settings.set('_fee_reverse_calc', True)
+    assert prov.calculate_fee(Decimal('100.00')) == Decimal('5.26')
+
+
+@pytest.mark.django_db
+def test_payment_fee_reverse_percent_and_abs(event):
+    prov = DummyPaymentProvider(event)
+    prov.settings.set('_fee_abs', Decimal('0.30'))
+    prov.settings.set('_fee_percent', Decimal('2.90'))
+    prov.settings.set('_fee_reverse_calc', True)
+    assert prov.calculate_fee(Decimal('100.00')) == Decimal('3.30')
+
+
+@pytest.mark.django_db
+def test_payment_fee_reverse_percent_and_abs_default(event):
+    prov = DummyPaymentProvider(event)
+    prov.settings.set('_fee_abs', Decimal('0.30'))
+    prov.settings.set('_fee_percent', Decimal('2.90'))
+    assert prov.calculate_fee(Decimal('100.00')) == Decimal('3.30')

src/tests/testdummy/payment.py

@@ -0,0 +1,19 @@
+import logging
+
+from pretix.base.payment import BasePaymentProvider
+
+logger = logging.getLogger('tests.testdummy.ticketoutput')
+
+
+class DummyPaymentProvider(BasePaymentProvider):
+    identifier = 'testdummy'
+    verbose_name = 'Test dummy'
+
+    def order_pending_render(self, request, order) -> str:
+        pass
+
+    def payment_is_valid_session(self, request) -> bool:
+        pass
+
+    def checkout_confirm_render(self, request) -> str:
+        pass

src/tests/testdummy/signals.py

@@ -1,9 +1,17 @@
 from django.dispatch import receiver
 
-from pretix.base.signals import register_ticket_outputs
+from pretix.base.signals import (
+    register_payment_providers, register_ticket_outputs,
+)
 
 
 @receiver(register_ticket_outputs, dispatch_uid="output_dummy")
 def register_ticket_outputs(sender, **kwargs):
     from .ticketoutput import DummyTicketOutput
     return DummyTicketOutput
+
+
+@receiver(register_payment_providers, dispatch_uid="payment_dummy")
+def register_ticket_outputs(sender, **kwargs):
+    from .payment import DummyPaymentProvider
+    return DummyPaymentProvider