Fixed #24652 -- Disallowed query execution in SimpleTestCase subclasses.

Thanks to Tim and Anssi for the review.

django/test/testcases.py

@@ -140,6 +140,19 @@ class _AssertTemplateNotUsedContext(_AssertTemplateUsedContext):
         return '%s was rendered.' % self.template_name
 
 
+class _CursorFailure(object):
+    def __init__(self, cls_name, wrapped):
+        self.cls_name = cls_name
+        self.wrapped = wrapped
+
+    def __call__(self):
+        raise AssertionError(
+            "Database queries aren't allowed in SimpleTestCase. "
+            "Either use TestCase or TransactionTestCase to ensure proper test isolation or "
+            "set %s.allow_database_queries to True to silence this failure." % self.cls_name
+        )
+
+
 class SimpleTestCase(unittest.TestCase):
 
     # The class we'll use for the test client self.client.
@@ -148,6 +161,10 @@ class SimpleTestCase(unittest.TestCase):
     _overridden_settings = None
     _modified_settings = None
 
+    # Tests shouldn't be allowed to query the database since
+    # this base class doesn't enforce any isolation.
+    allow_database_queries = False
+
     @classmethod
     def setUpClass(cls):
         super(SimpleTestCase, cls).setUpClass()
@@ -157,9 +174,17 @@ class SimpleTestCase(unittest.TestCase):
         if cls._modified_settings:
             cls._cls_modified_context = modify_settings(cls._modified_settings)
             cls._cls_modified_context.enable()
+        if not cls.allow_database_queries:
+            for alias in connections:
+                connection = connections[alias]
+                connection.cursor = _CursorFailure(cls.__name__, connection.cursor)
 
     @classmethod
     def tearDownClass(cls):
+        if not cls.allow_database_queries:
+            for alias in connections:
+                connection = connections[alias]
+                connection.cursor = connection.cursor.wrapped
         if hasattr(cls, '_cls_modified_context'):
             cls._cls_modified_context.disable()
             delattr(cls, '_cls_modified_context')
@@ -777,6 +802,10 @@ class TransactionTestCase(SimpleTestCase):
     # This can be slow; this flag allows enabling on a per-case basis.
     serialized_rollback = False
 
+    # Since tests will be wrapped in a transaction, or serialized if they
+    # are not available, we allow queries to be run.
+    allow_database_queries = True
+
     def _pre_setup(self):
         """Performs any pre-test setup. This includes:
 

docs/releases/1.9.txt

@@ -505,6 +505,12 @@ Miscellaneous
 * The ``django.contrib.sites.models.Site.domain`` field was changed to be
   :attr:`~django.db.models.Field.unique`.
 
+* In order to enforce test isolation, database queries are not allowed
+  by default in :class:`~django.test.SimpleTestCase` tests anymore. You
+  can disable this behavior by setting the
+  :attr:`~django.test.SimpleTestCase.allow_database_queries` class attribute
+  to ``True`` on your test class.
+
 .. _deprecated-features-1.9:
 
 Features deprecated in 1.9

docs/topics/testing/tools.txt

@@ -606,6 +606,17 @@ features like:
 then you should use :class:`~django.test.TransactionTestCase` or
 :class:`~django.test.TestCase` instead.
 
+.. attribute:: SimpleTestCase.allow_database_queries
+
+    .. versionadded:: 1.9
+
+    :class:`~SimpleTestCase` disallows database queries by default. This
+    helps to avoid executing write queries which will affect other tests
+    since each ``SimpleTestCase`` test isn't run in a transaction. If you
+    aren't concerned about this problem, you can disable this behavior by
+    setting the ``allow_database_queries`` class attribute to ``True`` on
+    your test class.
+
 ``SimpleTestCase`` inherits from ``unittest.TestCase``.
 
 .. warning::

tests/test_utils/tests.py

@@ -914,3 +914,21 @@ class OverrideSettingsTests(TestCase):
         with self.settings(STATICFILES_DIRS=[test_path]):
             finder = get_finder('django.contrib.staticfiles.finders.FileSystemFinder')
             self.assertIn(expected_location, finder.locations)
+
+
+class DisallowedDatabaseQueriesTests(SimpleTestCase):
+    def test_disallowed_database_queries(self):
+        expected_message = (
+            "Database queries aren't allowed in SimpleTestCase. "
+            "Either use TestCase or TransactionTestCase to ensure proper test isolation or "
+            "set DisallowedDatabaseQueriesTests.allow_database_queries to True to silence this failure."
+        )
+        with self.assertRaisesMessage(AssertionError, expected_message):
+            Car.objects.first()
+
+
+class AllowedDatabaseQueriesTests(SimpleTestCase):
+    allow_database_queries = True
+
+    def test_allowed_database_queries(self):
+        Car.objects.first()