Use inspect instead of TypeError for backwards-compatible APIs

src/pretix/base/services/mail.py

@@ -1,3 +1,4 @@
+import inspect
 import logging
 import smtplib
 import warnings
@@ -177,9 +178,9 @@ def mail(email: str, subject: str, template: Union[str, LazyI18nString],
             body_plain += "\r\n"
 
         try:
-            try:
+            if 'position' in inspect.signature(renderer.render).parameters:
                 body_html = renderer.render(content_plain, signature, str(subject), order, position)
-            except TypeError:
+            else:
                 # Backwards compatibility
                 warnings.warn('E-mail renderer called without position argument because position argument is not '
                               'supported.',

src/pretix/presale/checkoutflow.py

@@ -1,3 +1,4 @@
+import inspect
 from decimal import Decimal
 
 from django.conf import settings
@@ -513,9 +514,9 @@ class PaymentStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
             if not provider.is_enabled or not self._is_allowed(provider, self.request):
                 continue
             fee = provider.calculate_fee(self._total_order_value)
-            try:
+            if 'total' in inspect.signature(provider.payment_form_render).parameters:
                 form = provider.payment_form_render(self.request, self._total_order_value + fee)
-            except TypeError:
+            else:
                 form = provider.payment_form_render(self.request)
             providers.append({
                 'provider': provider,

src/pretix/presale/views/order.py

@@ -1,3 +1,4 @@
+import inspect
 import mimetypes
 import os
 from decimal import Decimal
@@ -306,9 +307,9 @@ class OrderPaymentStart(EventViewMixin, OrderDetailMixin, TemplateView):
 
     @cached_property
     def form(self):
-        try:
+        if 'total' in inspect.signature(self.payment.payment_provider.payment_form_render).parameters:
             return self.payment.payment_provider.payment_form_render(self.request, self.payment.amount)
-        except TypeError:
+        else:
             return self.payment.payment_provider.payment_form_render(self.request)
 
     @cached_property
@@ -521,9 +522,9 @@ class OrderPayChangeMethod(EventViewMixin, OrderDetailMixin, TemplateView):
                 continue
             current_fee = sum(f.value for f in self.open_fees) or Decimal('0.00')
             fee = provider.calculate_fee(pending_sum - current_fee)
-            try:
+            if 'total' in inspect.signature(provider.payment_form_render).parameters:
                 form = provider.payment_form_render(self.request, abs(pending_sum + fee - current_fee))
-            except TypeError:
+            else:
                 form = provider.payment_form_render(self.request)
             providers.append({
                 'provider': provider,