Fixed #24265 -- Preserved template backend loading exceptions.

                    "Could not find config for '{}' "

                    "in settings.TEMPLATES".format(alias))

            # If importing or initializing the backend raises an exception,

            # self._engines[alias] isn't set and this code may get executed

            # again, so we must preserve the original params. See #24265.

            params = params.copy()

            backend = params.pop('BACKEND')

            engine_cls = import_string(backend)

            engine = engine_cls(params)

from django.core.exceptions import ImproperlyConfigured

from django.template import engines

from django.test import SimpleTestCase, override_settings

class TemplateStringsTests(SimpleTestCase):

    @override_settings(TEMPLATES=[{

        'BACKEND': 'raise.import.error',

    }])

    def test_backend_import_error(self):

        """

        Failing to import a backend keeps raising the original import error.

        Regression test for #24265.

        """

        with self.assertRaises(ImportError):

            engines.all()

        with self.assertRaises(ImportError):

            engines.all()

    @override_settings(TEMPLATES=[{

        'BACKEND': 'django.template.backends.django.DjangoTemplates',

        # Incorrect: APP_DIRS and loaders are mutually incompatible.

        'APP_DIRS': True,

        'OPTIONS': {'loaders': []},

    }])

    def test_backend_improperly_configured(self):

        """

        Failing to initialize a backend keeps raising the original exception.

        Regression test for #24265.

        """

        with self.assertRaises(ImproperlyConfigured):

            engines.all()

        with self.assertRaises(ImproperlyConfigured):

            engines.all()