Fixed #27032 -- Prevented setup_test_environment() from being called twice.

django/test/utils.py

@@ -99,6 +99,13 @@ def setup_test_environment():
     Perform global pre-test setup, such as installing the instrumented template
     renderer and setting the email backend to the locmem email backend.
     """
+    if hasattr(Template, '_original_render'):
+        # Executing this function twice would overwrite the saved values.
+        raise RuntimeError(
+            "setup_test_environment() was already called and can't be called "
+            "again without first calling teardown_test_environment()."
+        )
+
     Template._original_render = Template._render
     Template._render = instrumented_test_render
 

docs/releases/1.11.txt

@@ -364,6 +364,11 @@ Miscellaneous
   :meth:`~django.db.models.fields.related.RelatedManager.set` now
   clear the ``prefetch_related()`` cache.
 
+* To prevent possible loss of saved settings,
+  :func:`~django.test.utils.setup_test_environment` now raises an exception if
+  called a second time before calling
+  :func:`~django.test.utils.teardown_test_environment`.
+
 .. _deprecated-features-1.11:
 
 Features deprecated in 1.11

tests/test_utils/tests.py

@@ -20,6 +20,7 @@ from django.test import (
 from django.test.html import HTMLParseError, parse_html
 from django.test.utils import (
     CaptureQueriesContext, isolate_apps, override_settings,
+    setup_test_environment,
 )
 from django.urls import NoReverseMatch, reverse
 from django.utils import six
@@ -864,6 +865,13 @@ class SecondUrls:
     urlpatterns = [url(r'second/$', empty_response, name='second')]
 
 
+class SetupTestEnvironmentTests(SimpleTestCase):
+
+    def test_setup_test_environment_calling_more_than_once(self):
+        with self.assertRaisesMessage(RuntimeError, "setup_test_environment() was already called"):
+            setup_test_environment()
+
+
 class OverrideSettingsTests(SimpleTestCase):
 
     # #21518 -- If neither override_settings nor a setting_changed receiver