Fixed #26365 -- Added a system check to ensure "string_is_invalid" is a string.

django/core/checks/templates.py

 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+import copy
+
 from django.conf import settings
+from django.utils import six
 
 from . import Error, Tags, register
 
@@ -10,6 +13,10 @@ E001 = Error(
     "in OPTIONS. Either remove APP_DIRS or remove the 'loaders' option.",
     id='templates.E001',
 )
+E002 = Error(
+    "'string_if_invalid' in TEMPLATES OPTIONS must be a string but got: {} ({}).",
+    id="templates.E002",
+)
 
 
 @register(Tags.templates)
@@ -21,3 +28,15 @@ def check_setting_app_dirs_loaders(app_configs, **kwargs):
         if 'loaders' in conf.get('OPTIONS', {}):
             passed_check = False
     return [] if passed_check else [E001]
+
+
+@register(Tags.templates)
+def check_string_if_invalid_is_string(app_configs, **kwargs):
+    errors = []
+    for conf in settings.TEMPLATES:
+        string_if_invalid = conf.get('OPTIONS', {}).get('string_if_invalid', '')
+        if not isinstance(string_if_invalid, six.string_types):
+            error = copy.copy(E002)
+            error.msg = error.msg.format(string_if_invalid, type(string_if_invalid).__name__)
+            errors.append(error)
+    return errors

docs/ref/checks.txt

@@ -584,6 +584,9 @@ configured:
 * **templates.E001**: You have ``'APP_DIRS': True`` in your
   :setting:`TEMPLATES` but also specify ``'loaders'`` in ``OPTIONS``. Either
   remove ``APP_DIRS`` or remove the ``'loaders'`` option.
+* **templates.E002**: ``string_if_invalid`` in :setting:`TEMPLATES`
+  :setting:`OPTIONS <TEMPLATES-OPTIONS>` must be a string but got: ``{value}``
+  (``{type}``).
 
 Caches
 ------

tests/check_framework/test_templates.py

-from copy import deepcopy
+from copy import copy, deepcopy
 
-from django.core.checks.templates import E001
+from django.core.checks.templates import E001, E002
 from django.test import SimpleTestCase
 from django.test.utils import override_settings
 
@@ -39,3 +39,58 @@ class CheckTemplateSettingsAppDirsTest(SimpleTestCase):
         del TEMPLATES[0]['OPTIONS']['loaders']
         with self.settings(TEMPLATES=TEMPLATES):
             self.assertEqual(self.func(None), [])
+
+
+class CheckTemplateStringIfInvalidTest(SimpleTestCase):
+    TEMPLATES_STRING_IF_INVALID = [
+        {
+            'BACKEND': 'django.template.backends.django.DjangoTemplates',
+            'OPTIONS': {
+                'string_if_invalid': False,
+            },
+        },
+        {
+            'BACKEND': 'django.template.backends.django.DjangoTemplates',
+            'OPTIONS': {
+                'string_if_invalid': 42,
+            },
+        },
+    ]
+
+    @classmethod
+    def setUpClass(cls):
+        super(CheckTemplateStringIfInvalidTest, cls).setUpClass()
+        cls.error1 = copy(E002)
+        cls.error2 = copy(E002)
+        string_if_invalid1 = cls.TEMPLATES_STRING_IF_INVALID[0]['OPTIONS']['string_if_invalid']
+        string_if_invalid2 = cls.TEMPLATES_STRING_IF_INVALID[1]['OPTIONS']['string_if_invalid']
+        cls.error1.msg = cls.error1.msg.format(string_if_invalid1, type(string_if_invalid1).__name__)
+        cls.error2.msg = cls.error2.msg.format(string_if_invalid2, type(string_if_invalid2).__name__)
+
+    @property
+    def func(self):
+        from django.core.checks.templates import check_string_if_invalid_is_string
+        return check_string_if_invalid_is_string
+
+    @override_settings(TEMPLATES=TEMPLATES_STRING_IF_INVALID)
+    def test_string_if_invalid_not_string(self):
+        self.assertEqual(self.func(None), [self.error1, self.error2])
+
+    def test_string_if_invalid_first_is_string(self):
+        TEMPLATES = deepcopy(self.TEMPLATES_STRING_IF_INVALID)
+        TEMPLATES[0]['OPTIONS']['string_if_invalid'] = 'test'
+        with self.settings(TEMPLATES=TEMPLATES):
+            self.assertEqual(self.func(None), [self.error2])
+
+    def test_string_if_invalid_both_are_strings(self):
+        TEMPLATES = deepcopy(self.TEMPLATES_STRING_IF_INVALID)
+        TEMPLATES[0]['OPTIONS']['string_if_invalid'] = 'test'
+        TEMPLATES[1]['OPTIONS']['string_if_invalid'] = 'test'
+        with self.settings(TEMPLATES=TEMPLATES):
+            self.assertEqual(self.func(None), [])
+
+    def test_string_if_invalid_not_specified(self):
+        TEMPLATES = deepcopy(self.TEMPLATES_STRING_IF_INVALID)
+        del TEMPLATES[1]['OPTIONS']['string_if_invalid']
+        with self.settings(TEMPLATES=TEMPLATES):
+            self.assertEqual(self.func(None), [self.error1])