Fixed #24235 -- Removed is_usable attribute from template loaders.

django/template/engine.py

@@ -121,18 +121,7 @@ class Engine(object):
                     "instead of django.template.loaders.base.Loader. " %
                     loader, RemovedInDjango20Warning, stacklevel=2)
 
-            loader_instance = loader_class(*args)
-
-            if not loader_instance.is_usable:
-                warnings.warn(
-                    "Your template loaders configuration includes %r, but "
-                    "your Python installation doesn't support that type of "
-                    "template loading. Consider removing that line from "
-                    "your settings." % loader)
-                return None
-            else:
-                return loader_instance
-
+            return loader_class(*args)
         else:
             raise ImproperlyConfigured(
                 "Invalid value in template loaders configuration: %r" % loader)

django/template/loaders/app_directories.py

@@ -14,7 +14,6 @@ from .base import Loader as BaseLoader
 
 
 class Loader(BaseLoader):
-    is_usable = True
 
     def get_template_sources(self, template_name, template_dirs=None):
         """

django/template/loaders/base.py

@@ -2,7 +2,6 @@ from django.template.base import Template, TemplateDoesNotExist
 
 
 class Loader(object):
-    is_usable = False
     # Only used to raise a deprecation warning. Remove in Django 2.0.
     _accepts_engine_in_init = True
 

django/template/loaders/cached.py

@@ -11,7 +11,6 @@ from .base import Loader as BaseLoader
 
 
 class Loader(BaseLoader):
-    is_usable = True
 
     def __init__(self, engine, loaders):
         self.template_cache = {}

django/template/loaders/eggs.py

@@ -14,7 +14,11 @@ from .base import Loader as BaseLoader
 
 
 class Loader(BaseLoader):
-    is_usable = resource_string is not None
+
+    def __init__(self, engine):
+        if resource_string is None:
+            raise RuntimeError("Setuptools must be installed to use the egg loader")
+        super(Loader, self).__init__(engine)
 
     def load_template_source(self, template_name, template_dirs=None):
         """
@@ -22,7 +26,6 @@ class Loader(BaseLoader):
 
         For every installed app, it tries to get the resource (app, template_name).
         """
-        if resource_string is not None:
         pkg_name = 'templates/' + template_name
         for app_config in apps.get_app_configs():
             try:

django/template/loaders/filesystem.py

@@ -12,7 +12,6 @@ from .base import Loader as BaseLoader
 
 
 class Loader(BaseLoader):
-    is_usable = True
 
     def get_template_sources(self, template_name, template_dirs=None):
         """
@@ -44,4 +43,3 @@ class Loader(BaseLoader):
             error_msg = ("Your template directories configuration is empty. "
                          "Change it to point to at least one template directory.")
         raise TemplateDoesNotExist(error_msg)
-    load_template_source.is_usable = True

django/template/loaders/locmem.py

@@ -8,7 +8,6 @@ from .base import Loader as BaseLoader
 
 
 class Loader(BaseLoader):
-    is_usable = True
 
     def __init__(self, engine, templates_dict):
         self.templates_dict = templates_dict

docs/releases/1.9.txt

@@ -182,6 +182,16 @@ Default settings that were tuples are now lists
 The default settings in ``django.conf.global_settings`` were a combination of
 lists and tuples. All settings that were formerly tuples are now lists.
 
+``is_usable`` attribute on template loaders is removed
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Django template loaders previously required an ``is_usable`` attribute to be
+defined. If a loader was configured in the template settings and this attribute
+was ``False``, the loader would be silently ignored. In practice, this was only
+used by the egg loader to detect if setuptools was installed. The ``is_usable``
+attribute is now removed and the egg loader instead fails at runtime if
+setuptools is not installed.
+
 Miscellaneous
 ~~~~~~~~~~~~~