[1.5.x] Fixed #20278 -- ensured .get() exceptions do not recurse infinitely

A regression caused by d5b93d3281 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.

Backpatch of 266c0bb23e

django/db/models/query.py

@@ -384,13 +384,11 @@ class QuerySet(object):
             return clone._result_cache[0]
         if not num:
             raise self.model.DoesNotExist(
-                "%s matching query does not exist. "
-                "Lookup parameters were %s" %
-                (self.model._meta.object_name, kwargs))
+                "%s matching query does not exist." %
+                self.model._meta.object_name)
         raise self.model.MultipleObjectsReturned(
-            "get() returned more than one %s -- it returned %s! "
-            "Lookup parameters were %s" %
-            (self.model._meta.object_name, num, kwargs))
+            "get() returned more than one %s -- it returned %s!" %
+            (self.model._meta.object_name, num))
 
     def create(self, **kwargs):
         """

tests/modeltests/basic/models.py

@@ -18,3 +18,11 @@ class Article(models.Model):
 
     def __str__(self):
         return self.headline
+
+@python_2_unicode_compatible
+class SelfRef(models.Model):
+    selfref = models.ForeignKey('self', null=True, blank=True,
+                                related_name='+')
+
+    def __str__(self):
+        return SelfRef.objects.get(selfref=self).pk

tests/modeltests/basic/tests.py

@@ -8,7 +8,7 @@ from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
 from django.utils.translation import ugettext_lazy
 
-from .models import Article
+from .models import Article, SelfRef
 
 
 class ModelTest(TestCase):
@@ -84,23 +84,14 @@ class ModelTest(TestCase):
         # parameters don't match any object.
         six.assertRaisesRegex(self,
             ObjectDoesNotExist,
-            "Article matching query does not exist. Lookup parameters were "
-            "{'id__exact': 2000}",
+            "Article matching query does not exist.",
             Article.objects.get,
             id__exact=2000,
         )
         # To avoid dict-ordering related errors check only one lookup
         # in single assert.
-        six.assertRaisesRegex(self,
+        self.assertRaises(
             ObjectDoesNotExist,
-            ".*'pub_date__year': 2005.*",
-            Article.objects.get,
-            pub_date__year=2005,
-            pub_date__month=8,
-        )
-        six.assertRaisesRegex(self,
-            ObjectDoesNotExist,
-            ".*'pub_date__month': 8.*",
             Article.objects.get,
             pub_date__year=2005,
             pub_date__month=8,
@@ -108,8 +99,7 @@ class ModelTest(TestCase):
 
         six.assertRaisesRegex(self,
             ObjectDoesNotExist,
-            "Article matching query does not exist. Lookup parameters were "
-            "{'pub_date__week_day': 6}",
+            "Article matching query does not exist.",
             Article.objects.get,
             pub_date__week_day=6,
         )
@@ -639,3 +629,8 @@ class ModelTest(TestCase):
         Article.objects.bulk_create([Article(headline=lazy, pub_date=datetime.now())])
         article = Article.objects.get()
         self.assertEqual(article.headline, notlazy)
+
+    def test_ticket_20278(self):
+        sr = SelfRef.objects.create()
+        with self.assertRaises(ObjectDoesNotExist):
+            SelfRef.objects.get(selfref=sr)