Fixed #35233 -- Moved template engine system checks to backend methods.

Thanks Adam Johnson for reviews.

django/core/checks/templates.py

@@ -1,75 +1,12 @@
-import copy
-from collections import defaultdict
-
-from django.conf import settings
-from django.template.backends.django import get_template_tag_modules
-
-from . import Error, Tags, Warning, register
-
-E001 = Error(
-    "You have 'APP_DIRS': True in your TEMPLATES but also specify 'loaders' "
-    "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",
-)
-W003 = Warning(
-    "{} is used for multiple template tag modules: {}",
-    id="templates.E003",
-)
+from . import Tags, register
 
 
 @register(Tags.templates)
-def check_setting_app_dirs_loaders(app_configs, **kwargs):
-    return (
-        [E001]
-        if any(
-            conf.get("APP_DIRS") and "loaders" in conf.get("OPTIONS", {})
-            for conf in settings.TEMPLATES
-        )
-        else []
-    )
+def check_templates(app_configs, **kwargs):
+    """Check all registered template engines."""
+    from django.template import engines
 
-
-@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, str):
-            error = copy.copy(E002)
-            error.msg = error.msg.format(
-                string_if_invalid, type(string_if_invalid).__name__
-            )
-            errors.append(error)
-    return errors
-
-
-@register(Tags.templates)
-def check_for_template_tags_with_the_same_name(app_configs, **kwargs):
-    errors = []
-    libraries = defaultdict(set)
-
-    for conf in settings.TEMPLATES:
-        custom_libraries = conf.get("OPTIONS", {}).get("libraries", {})
-        for module_name, module_path in custom_libraries.items():
-            libraries[module_name].add(module_path)
-
-    for module_name, module_path in get_template_tag_modules():
-        libraries[module_name].add(module_path)
-
-    for library_name, items in libraries.items():
-        if len(items) > 1:
-            errors.append(
-                Warning(
-                    W003.msg.format(
-                        repr(library_name),
-                        ", ".join(repr(item) for item in sorted(items)),
-                    ),
-                    id=W003.id,
-                )
-            )
-
+    for engine in engines.all():
+        errors.extend(engine.check())
     return errors

django/template/backends/base.py

@@ -23,6 +23,9 @@ class BaseEngine:
                 "Unknown parameters: {}".format(", ".join(params))
             )
 
+    def check(self, **kwargs):
+        return []
+
     @property
     def app_dirname(self):
         raise ImproperlyConfigured(

django/template/backends/django.py

@@ -1,8 +1,10 @@
+from collections import defaultdict
 from importlib import import_module
 from pkgutil import walk_packages
 
 from django.apps import apps
 from django.conf import settings
+from django.core.checks import Error, Warning
 from django.template import TemplateDoesNotExist
 from django.template.context import make_context
 from django.template.engine import Engine
@@ -25,6 +27,50 @@ class DjangoTemplates(BaseEngine):
         super().__init__(params)
         self.engine = Engine(self.dirs, self.app_dirs, **options)
 
+    def check(self, **kwargs):
+        return [
+            *self._check_string_if_invalid_is_string(),
+            *self._check_for_template_tags_with_the_same_name(),
+        ]
+
+    def _check_string_if_invalid_is_string(self):
+        value = self.engine.string_if_invalid
+        if not isinstance(value, str):
+            return [
+                Error(
+                    "'string_if_invalid' in TEMPLATES OPTIONS must be a string but "
+                    "got: %r (%s)." % (value, type(value)),
+                    obj=self,
+                    id="templates.E002",
+                )
+            ]
+        return []
+
+    def _check_for_template_tags_with_the_same_name(self):
+        libraries = defaultdict(set)
+
+        for module_name, module_path in get_template_tag_modules():
+            libraries[module_name].add(module_path)
+
+        for module_name, module_path in self.engine.libraries.items():
+            libraries[module_name].add(module_path)
+
+        errors = []
+
+        for library_name, items in libraries.items():
+            if len(items) > 1:
+                items = ", ".join(repr(item) for item in sorted(items))
+                errors.append(
+                    Warning(
+                        f"{library_name!r} is used for multiple template tag modules: "
+                        f"{items}",
+                        obj=self,
+                        id="templates.W003",
+                    )
+                )
+
+        return errors
+
     def from_string(self, template_code):
         return Template(self.engine.from_string(template_code), self)