Fix bug that lead to wrong payment amount when switching payment method to PayPal later

src/pretix/base/payment.py

@@ -1,7 +1,7 @@
 from collections import OrderedDict
 from datetime import date
 from decimal import Decimal
-from typing import Any, Dict
+from typing import Any, Dict, Union
 
 import pytz
 from django import forms
@@ -266,7 +266,7 @@ class BasePaymentProvider:
         """
         raise NotImplementedError()  # NOQA
 
-    def checkout_prepare(self, request: HttpRequest, cart: Dict[str, Any]) -> "bool|str":
+    def checkout_prepare(self, request: HttpRequest, cart: Dict[str, Any]) -> Union[bool, str]:
         """
         Will be called after the user selects this provider as his payment method.
         If you provided a form to the user to enter payment data, this method should
@@ -394,14 +394,14 @@ class BasePaymentProvider:
         """
         return False
 
-    def retry_prepare(self, request: HttpRequest, order: Order) -> "bool|str":
+    def retry_prepare(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Deprecated, use order_prepare instead
         """
         raise DeprecationWarning('retry_prepare is deprecated, use order_prepare instead')
         return self.order_prepare(request, order)
 
-    def order_prepare(self, request: HttpRequest, order: Order) -> "bool|str":
+    def order_prepare(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Will be called if the user retries to pay an unpaid order (after the user filled in
         e.g. the form returned by :py:meth:`payment_form`) or if the user changes the payment
@@ -409,6 +409,10 @@ class BasePaymentProvider:
 
         It should return and report errors the same way as :py:meth:`checkout_prepare`, but
         receives an ``Order`` object instead of a cart object.
+
+        Note: The ``Order`` object given to this method might be different from the version
+        stored in the database as it's total will already contain the payment fee for the
+        new payment method.
         """
         form = self.payment_form(request)
         if form.is_valid():
@@ -458,7 +462,7 @@ class BasePaymentProvider:
         return '<div class="alert alert-warning">%s</div>' % _('The money can not be automatically refunded, '
                                                                'please transfer the money back manually.')
 
-    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> "bool|str":
+    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Will be called if the event administrator confirms the refund.
 
@@ -523,7 +527,7 @@ class FreeOrderProvider(BasePaymentProvider):
     def order_control_refund_render(self, order: Order) -> str:
         return ''
 
-    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> "bool|str":
+    def order_control_refund_perform(self, request: HttpRequest, order: Order) -> Union[bool, str]:
         """
         Will be called if the event administrator confirms the refund.