Fixed #29642 -- Added check for arguments of custom error handler views.

django/urls/resolvers.py

@@ -6,13 +6,14 @@ a string) and returns a ResolverMatch object which provides access to all
 attributes of the resolved URL match.
 """
 import functools
+import inspect
 import re
 import threading
 from importlib import import_module
 from urllib.parse import quote
 
 from django.conf import settings
-from django.core.checks import Warning
+from django.core.checks import Error, Warning
 from django.core.checks.urls import check_resolver
 from django.core.exceptions import ImproperlyConfigured
 from django.utils.datastructures import MultiValueDict
@@ -392,10 +393,33 @@ class URLResolver:
         )
 
     def check(self):
-        warnings = []
+        messages = []
         for pattern in self.url_patterns:
-            warnings.extend(check_resolver(pattern))
-        return warnings or self.pattern.check()
+            messages.extend(check_resolver(pattern))
+        messages.extend(self._check_custom_error_handlers())
+        return messages or self.pattern.check()
+
+    def _check_custom_error_handlers(self):
+        messages = []
+        # All handlers take (request, exception) arguments except handler500
+        # which takes (request).
+        for status_code, num_parameters in [(400, 2), (403, 2), (404, 2), (500, 1)]:
+            handler, param_dict = self.resolve_error_handler(status_code)
+            signature = inspect.signature(handler)
+            args = [None] * num_parameters
+            try:
+                signature.bind(*args)
+            except TypeError:
+                msg = (
+                    "The custom handler{status_code} view '{path}' does not "
+                    "take the correct number of arguments ({args})."
+                ).format(
+                    status_code=status_code,
+                    path=handler.__module__ + '.' + handler.__qualname__,
+                    args='request, exception' if num_parameters == 2 else 'request',
+                )
+                messages.append(Error(msg, id='urls.E007'))
+        return messages
 
     def _populate(self):
         # Short-circuit if called recursively in this thread to prevent

docs/ref/checks.txt

@@ -457,6 +457,8 @@ The following checks are performed on your URL configuration:
   able to reverse all URLs in this namespace.
 * **urls.E006**: The :setting:`MEDIA_URL`/ :setting:`STATIC_URL` setting must
   end with a slash.
+* **urls.E007**: The custom ``handlerXXX`` view ``'path.to.view'`` does not
+  take the correct number of arguments (…).
 
 ``contrib`` app checks
 ======================

tests/check_framework/test_urls.py

@@ -1,5 +1,5 @@
 from django.conf import settings
-from django.core.checks.messages import Warning
+from django.core.checks.messages import Error, Warning
 from django.core.checks.urls import (
     E006, check_url_config, check_url_namespaces_unique, check_url_settings,
     get_warning_for_invalid_pattern,
@@ -165,6 +165,28 @@ class UpdatedToPathTests(SimpleTestCase):
         self.assertIn(expected_msg, warning.msg)
 
 
+class CheckCustomErrorHandlersTests(SimpleTestCase):
+
+    @override_settings(ROOT_URLCONF='check_framework.urls.bad_error_handlers')
+    def test_bad_handlers(self):
+        result = check_url_config(None)
+        self.assertEqual(len(result), 4)
+        for code, num_params, error in zip([400, 403, 404, 500], [2, 2, 2, 1], result):
+            with self.subTest('handler{}'.format(code)):
+                self.assertEqual(error, Error(
+                    "The custom handler{} view "
+                    "'check_framework.urls.bad_error_handlers.bad_handler' "
+                    "does not take the correct number of arguments (request{})."
+                    .format(code, ', exception' if num_params == 2 else ''),
+                    id='urls.E007',
+                ))
+
+    @override_settings(ROOT_URLCONF='check_framework.urls.good_error_handlers')
+    def test_good_handlers(self):
+        result = check_url_config(None)
+        self.assertEqual(result, [])
+
+
 class CheckURLSettingsTests(SimpleTestCase):
 
     @override_settings(STATIC_URL='a/', MEDIA_URL='b/')

tests/check_framework/urls/bad_error_handlers.py

@@ -0,0 +1,10 @@
+urlpatterns = []
+
+handler400 = __name__ + '.bad_handler'
+handler403 = __name__ + '.bad_handler'
+handler404 = __name__ + '.bad_handler'
+handler500 = __name__ + '.bad_handler'
+
+
+def bad_handler():
+    pass

tests/check_framework/urls/good_error_handlers.py

@@ -0,0 +1,10 @@
+urlpatterns = []
+
+handler400 = __name__ + '.good_handler'
+handler403 = __name__ + '.good_handler'
+handler404 = __name__ + '.good_handler'
+handler500 = __name__ + '.good_handler'
+
+
+def good_handler(request, exception=None, foo='bar'):
+    pass