_setup: do not call django.setup() if settings are configured already

Without this, using `configurations.setup()` after Django has been setup
already re-triggers the logging configuration, which then causes e.g.
pytest's caplog to not work anymore.

.gitignore

@@ -1,4 +1,4 @@
-.coverage.*
+.coverage
 coverage.xml
 sitecustomize.py
 docs/_build

configurations/__init__.py

@@ -11,8 +11,10 @@ def _setup():
 
     importer.install()
 
-    import django
-    django.setup()
+    from django.apps import apps
+    if not apps.ready:
+        import django
+        django.setup()
 
 
 def load_ipython_extension(ipython):

tests/setup_test.py

@@ -0,0 +1,17 @@
+"""Used by tests to ensure logging is kept when calling setup() twice."""
+
+try:
+    from unittest import mock
+except ImportError:
+    from mock import mock
+
+import configurations
+
+print('setup_1')
+configurations.setup()
+
+with mock.patch('django.setup', side_effect=Exception('setup called twice')):
+    print('setup_2')
+    configurations.setup()
+
+print('setup_done')

tests/test_main.py

@@ -105,3 +105,16 @@ class MainTests(TestCase):
         proc = subprocess.Popen(['django-cadmin', 'runserver', '--help'],
                                 stdout=subprocess.PIPE)
         self.assertIn('--configuration', proc.communicate()[0].decode('utf-8'))
+
+    def test_django_setup_only_called_once(self):
+        proc = subprocess.Popen(
+            [sys.executable, os.path.join(os.path.dirname(__file__),
+                                          'setup_test.py')],
+            stdout=subprocess.PIPE)
+        res = proc.communicate()
+        stdout = res[0].decode('utf-8')
+
+        self.assertIn('setup_1', stdout)
+        self.assertIn('setup_2', stdout)
+        self.assertIn('setup_done', stdout)
+        self.assertEqual(proc.returncode, 0)

tox.ini

@@ -25,11 +25,12 @@ deps =
     dj20: django>=2.0a1,<2.1
     dj21: django>=2.1a1,<2.2
     djmaster: https://github.com/django/django/archive/master.tar.gz#egg=django
+    py27,pypy: mock
 
 commands =
     python --version
     coverage run {envbindir}/django-cadmin test -v2 {posargs:tests}
-    coverage combine
+    coverage combine . tests/docs
     coverage report -m --skip-covered
 
 [testenv:readme-py27]