Make plugin dependencies optional by allowing plugins to report errors

src/pretix/plugins/stripe/__init__.py

@@ -1,4 +1,5 @@
 from django.apps import AppConfig
+from django.utils.functional import cached_property
 from django.utils.translation import ugettext_lazy as _
 from pretix.base.plugins import PluginType
 
@@ -18,5 +19,14 @@ class StripeApp(AppConfig):
     def ready(self):
         from . import signals  # NOQA
 
+    @cached_property
+    def compatibility_errors(self):
+        errs = []
+        try:
+            import stripe
+        except ImportError:
+            errs.append("Python package 'stripe' is not installed.")
+        return errs
+
 
 default_app_config = 'pretix.plugins.stripe.StripeApp'

src/pretix/plugins/stripe/signals.py

@@ -5,17 +5,18 @@ from django.template.loader import get_template
 
 from pretix.base.signals import register_payment_providers
 
-from .payment import Stripe
 from pretix.presale.signals import html_head
 
 
 @receiver(register_payment_providers)
 def register_payment_provider(sender, **kwargs):
+    from .payment import Stripe
     return Stripe
 
 
 @receiver(html_head)
 def html_head_presale(sender, request=None, **kwargs):
+    from .payment import Stripe
     provider = Stripe(sender)
     url = resolve(request.path_info)
     if provider.is_enabled and "checkout.payment" in url.url_name: