handle payment provider (de-)selection and partial payments

src/pretix/base/services/cart.py

@@ -1605,6 +1605,7 @@ def add_payment_to_cart_session(cart_session, provider, min_value: Decimal=None,
         'max_value': str(max_value) if max_value is not None else None,
         'info_data': info_data or {},
     })
+    cart_session['payments_postpone'] = False
 
 
 def add_payment_to_cart(request, provider, min_value: Decimal=None, max_value: Decimal=None, info_data: dict=None):

src/pretix/presale/checkoutflow.py

@@ -1440,10 +1440,12 @@ class PaymentStep(CartMixin, TemplateFlowStep):
         ctx['providers'] = self.provider_forms
         ctx['show_fees'] = any(p['fee'] for p in self.provider_forms)
 
-        if len(self.provider_forms) == 1:
-            ctx['selected'] = self.provider_forms[0]['provider'].identifier
-        elif 'payment' in self.request.POST:
+        if 'payment' in self.request.POST:
             ctx['selected'] = self.request.POST['payment']
+        elif self.cart_session.get('payments_postpone') and self._allow_postpone:
+            ctx['selected'] = ''
+        elif len(self.provider_forms) == 1:
+            ctx['selected'] = self.provider_forms[0]['provider'].identifier
         elif self.single_use_payment:
             ctx['selected'] = self.single_use_payment['provider']
         else:

src/pretix/presale/templates/pretixpresale/event/checkout_payment.html

@@ -135,7 +135,9 @@
                         {% trans "Not sure yet? You can complete your order first and then select a payment method later." %}
                         <br>
                         <span class="text-muted">
-                            {% if payment_deadline_minutes %}
+                            {% if current_payments %}
+                                {% trans "To do so, please first remove the payment methods you already selected above." %}
+                            {% elif payment_deadline_minutes %}
                                 {% blocktrans trimmed with minutes=payment_deadline_minutes %}
                                     Your payment needs to be completed within {{ minutes }} minutes.
                                 {% endblocktrans %}
@@ -147,7 +149,8 @@
                         </span>
                     </div>
                     <div class="col-md-3 col-xs-12 text-right flip">
-                        <button name="postpone" value="on" class="btn btn-primary">
+                        <button name="postpone" value="on" class="btn btn-primary"
+                                {% if current_payments %}disabled{% endif %}>
                             {% trans "Proceed without selection" %}
                         </button>
                     </div>

src/tests/presale/test_checkout.py

@@ -2405,6 +2405,64 @@ class CheckoutTestCase(BaseCheckoutTestCase, TimemachineTestMixin, TestCase):
             o = Order.objects.last()
             assert not o.payments.exists()
 
+    def test_payment_postpone_cleared_on_selection(self):
+        self.event.settings.set('payment_banktransfer__enabled', True)
+        self.event.settings.payment_choice_postpone_allowed_channels = ['web']
+        with scopes_disabled():
+            CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=23, expires=now() + timedelta(minutes=10)
+            )
+
+        response = self.client.post('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), {
+            'postpone': 'on',
+        }, follow=False)
+        self.assertRedirects(response, '/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+        assert self.client.session['carts'][self.session_key].get('payments_postpone')
+
+        # The only available provider must not be preselected while the choice is postponed
+        response = self.client.get('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.content.decode(), "lxml")
+        self.assertEqual(len(doc.select('input[name="payment"]')), 1)
+        self.assertEqual(len(doc.select('input[name="payment"][checked]')), 0)
+
+        # Selecting a payment method takes the order out of the postponed state again
+        response = self.client.post('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), {
+            'payment': 'banktransfer',
+        }, follow=False)
+        self.assertRedirects(response, '/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+        assert not self.client.session['carts'][self.session_key].get('payments_postpone')
+
+    def test_payment_postpone_disabled_with_partial_payment(self):
+        self.event.settings.set('payment_banktransfer__enabled', True)
+        self.event.settings.payment_choice_postpone_allowed_channels = ['web']
+        gc = self.orga.issued_gift_cards.create(currency="EUR")
+        gc.transactions.create(value=20, acceptor=self.orga)
+        with scopes_disabled():
+            CartPosition.objects.create(
+                event=self.event, cart_id=self.session_key, item=self.ticket,
+                price=23, expires=now() + timedelta(minutes=10)
+            )
+
+        response = self.client.get('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), follow=True)
+        doc = BeautifulSoup(response.content.decode(), "lxml")
+        self.assertEqual(len(doc.select('button[name="postpone"]')), 1)
+        self.assertEqual(len(doc.select('button[name="postpone"][disabled]')), 0)
+
+        # Apply a gift card that only covers part of the total
+        response = self.client.post('/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug), {
+            'payment': 'giftcard',
+            'payment_giftcard-code': gc.secret,
+        }, follow=True)
+        self.assertRedirects(response, '/%s/%s/checkout/payment/' % (self.orga.slug, self.event.slug),
+                             target_status_code=200)
+
+        # Postponing would silently drop the gift card, so it is no longer offered
+        doc = BeautifulSoup(response.content.decode(), "lxml")
+        self.assertEqual(len(doc.select('button[name="postpone"][disabled]')), 1)
+
     def test_premature_confirm(self):
         response = self.client.get('/%s/%s/checkout/confirm/' % (self.orga.slug, self.event.slug), follow=True)
         self.assertRedirects(response, '/%s/%s/?require_cookie=true' % (self.orga.slug, self.event.slug),