Fixed #23742 -- Added an option to reverse tests order.

This is useful for debugging side effects affecting tests that
are usually executed before a given test. Full suite and pair
tests sort cases more or less deterministically, thus some test
cross-dependencies are easier to reveal by reversing the order.

Thanks Preston Timmons for the review.

django/test/runner.py

@@ -20,7 +20,7 @@ class DiscoverRunner(object):
     reorder_by = (TestCase, SimpleTestCase)
 
     def __init__(self, pattern=None, top_level=None,
-                 verbosity=1, interactive=True, failfast=False, keepdb=False,
+                 verbosity=1, interactive=True, failfast=False, keepdb=False, reverse=False,
                  **kwargs):
 
         self.pattern = pattern
@@ -30,6 +30,7 @@ class DiscoverRunner(object):
         self.interactive = interactive
         self.failfast = failfast
         self.keepdb = keepdb
+        self.reverse = reverse
 
     @classmethod
     def add_arguments(cls, parser):
@@ -41,7 +42,10 @@ class DiscoverRunner(object):
             help='The test matching pattern. Defaults to test*.py.')
         parser.add_argument('-k', '--keepdb', action='store_true', dest='keepdb',
             default=False,
-            help='Preserve the test DB between runs. Defaults to False')
+            help='Preserves the test DB between runs.')
+        parser.add_argument('-r', '--reverse', action='store_true', dest='reverse',
+            default=False,
+            help='Reverses test cases order.')
 
     def setup_test_environment(self, **kwargs):
         setup_test_environment()
@@ -107,7 +111,7 @@ class DiscoverRunner(object):
         for test in extra_tests:
             suite.addTest(test)
 
-        return reorder_suite(suite, self.reorder_by)
+        return reorder_suite(suite, self.reorder_by, self.reverse)
 
     def setup_databases(self, **kwargs):
         return setup_databases(self.verbosity, self.interactive, self.keepdb, **kwargs)
@@ -213,7 +217,7 @@ def dependency_ordered(test_databases, dependencies):
     return ordered_test_databases
 
 
-def reorder_suite(suite, classes):
+def reorder_suite(suite, classes, reverse=False):
     """
     Reorders a test suite by test type.
 
@@ -221,30 +225,36 @@ def reorder_suite(suite, classes):
 
     All tests of type classes[0] are placed first, then tests of type
     classes[1], etc. Tests with no match in classes are placed last.
+
+    If `reverse` is True, tests within classes are sorted in opposite order,
+    but test classes are not reversed.
     """
     class_count = len(classes)
     suite_class = type(suite)
     bins = [suite_class() for i in range(class_count + 1)]
-    partition_suite(suite, classes, bins)
+    partition_suite(suite, classes, bins, reverse=reverse)
     for i in range(class_count):
         bins[0].addTests(bins[i + 1])
     return bins[0]
 
 
-def partition_suite(suite, classes, bins):
+def partition_suite(suite, classes, bins, reverse=False):
     """
     Partitions a test suite by test type. Also prevents duplicated tests.
 
     classes is a sequence of types
     bins is a sequence of TestSuites, one more than classes
+    reverse changes the ordering of tests within bins
 
     Tests of type classes[i] are added to bins[i],
     tests with no match found in classes are place in bins[-1]
     """
     suite_class = type(suite)
+    if reverse:
+        suite = reversed(tuple(suite))
     for test in suite:
         if isinstance(test, suite_class):
-            partition_suite(test, classes, bins)
+            partition_suite(test, classes, bins, reverse=reverse)
         else:
             for i in range(len(classes)):
                 if isinstance(test, classes[i]):

docs/internals/contributing/writing-code/unit-tests.txt

@@ -307,3 +307,15 @@ the first one:
 .. code-block:: bash
 
    $ ./runtests.py --pair basic.tests.ModelTest.test_eq queries transactions
+
+You can also try running any set of tests in reverse using the ``--reverse``
+option in order to verify that executing tests in a different order does not
+cause any trouble:
+
+.. code-block:: bash
+
+   $ ./runtests.py basic --reverse
+
+.. versionadded:: 1.8
+
+    The ``--reverse`` option was added.

docs/ref/django-admin.txt

@@ -1411,6 +1411,15 @@ test suite. If the test database does not exist, it will be created on the first
 run and then preserved for each subsequent run. Any unapplied migrations will also
 be applied to the test database before running the test suite.
 
+.. django-admin-option:: --reverse
+
+.. versionadded:: 1.8
+
+The ``--reverse`` option can be used to sort test cases in the opposite order.
+This may help in debugging tests that aren't properly isolated and have side
+effects. :ref:`Grouping by test class <order-of-tests>` is preserved when using
+this option.
+
 testserver <fixture fixture ...>
 --------------------------------
 

docs/releases/1.8.txt

@@ -471,8 +471,9 @@ Tests
 * The new :meth:`~django.test.SimpleTestCase.assertJSONNotEqual` assertion
   allows you to test that two JSON fragments are not equal.
 
-* Added the ability to preserve the test database by adding the
-  :djadminopt:`--keepdb` flag.
+* Added options to the :djadmin:`test` command to preserve the test database
+  (:djadminopt:`--keepdb`) and to run the test cases in reverse order
+  (:djadminopt:`--reverse`).
 
 * Added the :attr:`~django.test.Response.resolver_match` attribute to test
   client responses.

docs/topics/testing/advanced.txt

@@ -355,7 +355,7 @@ behavior. This class defines the ``run_tests()`` entry point, plus a
 selection of other methods that are used to by ``run_tests()`` to set up,
 execute and tear down the test suite.
 
-.. class:: DiscoverRunner(pattern='test*.py', top_level=None, verbosity=1, interactive=True, failfast=True, keepdb=False **kwargs)
+.. class:: DiscoverRunner(pattern='test*.py', top_level=None, verbosity=1, interactive=True, failfast=True, keepdb=False, reverse=False, **kwargs)
 
     ``DiscoverRunner`` will search for tests in any file matching ``pattern``.
 
@@ -381,6 +381,11 @@ execute and tear down the test suite.
     or create one if necessary. If ``False``, a new database will be created,
     prompting the user to remove the existing one, if present.
 
+    If ``reverse`` is ``True``, test cases will be executed in the opposite
+    order. This could be useful to debug tests that aren't properly isolated
+    and have side effects. :ref:`Grouping by test class <order-of-tests>` is
+    preserved when using this option.
+
     Django may, from time to time, extend the capabilities of the test runner
     by adding new arguments. The ``**kwargs`` declaration allows for this
     expansion. If you subclass ``DiscoverRunner`` or write your own test
@@ -397,7 +402,7 @@ execute and tear down the test suite.
         subclassed test runner to add options to the list of command-line
         options that the :djadmin:`test` command could use.
 
-        The ``keepdb`` argument was added.
+        The ``keepdb`` and the ``reverse`` arguments were added.
 
 Attributes
 ~~~~~~~~~~

docs/topics/testing/overview.txt

@@ -234,6 +234,12 @@ the Django test runner reorders tests in the following way:
     database by a given :class:`~django.test.TransactionTestCase` test, they
     must be updated to be able to run independently.
 
+.. versionadded:: 1.8
+
+    You may reverse the execution order inside groups by passing
+    :djadminopt:`--reverse` to the test command. This can help with ensuring
+    your tests are independent from each other.
+
 .. _test-case-serialized-rollback:
 
 Rollback emulation

tests/runtests.py

@@ -198,7 +198,7 @@ def teardown(state):
         setattr(settings, key, value)
 
 
-def django_tests(verbosity, interactive, failfast, keepdb, test_labels):
+def django_tests(verbosity, interactive, failfast, keepdb, reverse, test_labels):
     state = setup(verbosity, test_labels)
     extra_tests = []
 
@@ -212,6 +212,7 @@ def django_tests(verbosity, interactive, failfast, keepdb, test_labels):
         interactive=interactive,
         failfast=failfast,
         keepdb=keepdb,
+        reverse=reverse,
     )
     # Catch warnings thrown in test DB setup -- remove in Django 1.9
     with warnings.catch_warnings():
@@ -361,6 +362,9 @@ if __name__ == "__main__":
     parser.add_argument('--pair',
         help='Run the test suite in pairs with the named test to find problem '
              'pairs.')
+    parser.add_argument('--reverse', action='store_true', default=False,
+        help='Sort test suites and test cases in opposite order to debug '
+             'test side effects not apparent with normal execution lineup.')
     parser.add_argument('--liveserver',
         help='Overrides the default address where the live server (used with '
              'LiveServerTestCase) is expected to run from. The default value '
@@ -393,6 +397,6 @@ if __name__ == "__main__":
     else:
         failures = django_tests(options.verbosity, options.interactive,
                                 options.failfast, options.keepdb,
-                                options.modules)
+                                options.reverse, options.modules)
         if failures:
             sys.exit(bool(failures))

tests/test_discovery_sample2/tests.py

@@ -1,7 +1,57 @@
-from django.test import TestCase
+from unittest import TestCase
+
+from django.test import SimpleTestCase, TestCase as DjangoTestCase
 
 
-class Test(TestCase):
+class DjangoCase1(DjangoTestCase):
 
-    def test_sample(self):
+    def test_1(self):
+        pass
+
+    def test_2(self):
+        pass
+
+
+class DjangoCase2(DjangoTestCase):
+
+    def test_1(self):
+        pass
+
+    def test_2(self):
+        pass
+
+
+class SimpleCase1(SimpleTestCase):
+
+    def test_1(self):
+        pass
+
+    def test_2(self):
+        pass
+
+
+class SimpleCase2(SimpleTestCase):
+
+    def test_1(self):
+        pass
+
+    def test_2(self):
+        pass
+
+
+class UnittestCase1(TestCase):
+
+    def test_1(self):
+        pass
+
+    def test_2(self):
+        pass
+
+
+class UnittestCase2(TestCase):
+
+    def test_1(self):
+        pass
+
+    def test_2(self):
         pass

tests/test_runner/test_discover_runner.py

@@ -126,6 +126,35 @@ class DiscoverRunnerTest(TestCase):
             ["django.contrib.gis", "django.contrib.gis.tests.geo3d"]).countTestCases()
         self.assertEqual(single, dups)
 
+    def test_reverse(self):
+        """
+        Reverse should reorder tests while maintaining the grouping specified
+        by ``DiscoverRunner.reorder_by``.
+        """
+        runner = DiscoverRunner(reverse=True)
+        suite = runner.build_suite(
+            test_labels=('test_discovery_sample', 'test_discovery_sample2'))
+        self.assertIn('test_discovery_sample2', next(iter(suite)).id(),
+                      msg="Test labels should be reversed.")
+        suite = runner.build_suite(test_labels=('test_discovery_sample2',))
+        suite = tuple(suite)
+        self.assertIn('DjangoCase', suite[0].id(),
+                      msg="Test groups should not be reversed.")
+        self.assertIn('SimpleCase', suite[4].id(),
+                      msg="Test groups order should be preserved.")
+        self.assertIn('DjangoCase2', suite[0].id(),
+                      msg="Django test cases should be reversed.")
+        self.assertIn('SimpleCase2', suite[4].id(),
+                      msg="Simple test cases should be reversed.")
+        self.assertIn('UnittestCase2', suite[8].id(),
+                      msg="Unittest test cases should be reversed.")
+        self.assertIn('test_2', suite[0].id(),
+                      msg="Methods of Django cases should be reversed.")
+        self.assertIn('test_2', suite[4].id(),
+                      msg="Methods of simple cases should be reversed.")
+        self.assertIn('test_2', suite[8].id(),
+                      msg="Methods of unittest cases should be reversed.")
+
     def test_overrideable_test_suite(self):
         self.assertEqual(DiscoverRunner().test_suite, TestSuite)