Refactored listing template subdirectories in apps.

This change has the nice side effect of removing code that ran at import
time and depended on the app registry at module level -- a notorious
cause of AppRegistryNotReady exceptions.

django/template/loaders/app_directories.py

@@ -4,38 +4,16 @@ packages.
 """
 
 import io
-import os
-import sys
 
-from django.apps import apps
 from django.conf import settings
 from django.core.exceptions import SuspiciousFileOperation
 from django.template.base import TemplateDoesNotExist
+from django.template.utils import get_app_template_dirs
 from django.utils._os import safe_join
-from django.utils import six
 
 from .base import Loader as BaseLoader
 
 
-def calculate_app_template_dirs():
-    if six.PY2:
-        fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
-    app_template_dirs = []
-    for app_config in apps.get_app_configs():
-        if not app_config.path:
-            continue
-        template_dir = os.path.join(app_config.path, 'templates')
-        if os.path.isdir(template_dir):
-            if six.PY2:
-                template_dir = template_dir.decode(fs_encoding)
-            app_template_dirs.append(template_dir)
-    return tuple(app_template_dirs)
-
-
-# At compile time, cache the directories to search.
-app_template_dirs = calculate_app_template_dirs()
-
-
 class Loader(BaseLoader):
     is_usable = True
 
@@ -46,7 +24,7 @@ class Loader(BaseLoader):
         template dirs are excluded from the result set, for security reasons.
         """
         if not template_dirs:
-            template_dirs = app_template_dirs
+            template_dirs = get_app_template_dirs('templates')
         for template_dir in template_dirs:
             try:
                 yield safe_join(template_dir, template_name)

django/template/utils.py

@@ -0,0 +1,29 @@
+import os
+import sys
+
+from django.apps import apps
+from django.utils import lru_cache
+from django.utils import six
+
+
+@lru_cache.lru_cache()
+def get_app_template_dirs(dirname):
+    """
+    Return an iterable of paths of directories to load app templates from.
+
+    dirname is the name of the subdirectory containing templates inside
+    installed applications.
+    """
+    if six.PY2:
+        fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
+    template_dirs = []
+    for app_config in apps.get_app_configs():
+        if not app_config.path:
+            continue
+        template_dir = os.path.join(app_config.path, dirname)
+        if os.path.isdir(template_dir):
+            if six.PY2:
+                template_dir = template_dir.decode(fs_encoding)
+            template_dirs.append(template_dir)
+    # Immutable return value because it will be cached and shared by callers.
+    return tuple(template_dirs)

django/test/signals.py

@@ -39,9 +39,9 @@ def update_installed_apps(**kwargs):
         # Rebuild templatetags module cache.
         from django.template import base as mod
         mod.templatetags_modules = []
-        # Rebuild app_template_dirs cache.
+        # Rebuild get_app_template_dirs cache.
-        from django.template.loaders import app_directories as mod
+        from django.template.utils import get_app_template_dirs
-        mod.app_template_dirs = mod.calculate_app_template_dirs()
+        get_app_template_dirs.cache_clear()
         # Rebuild translations cache.
         from django.utils.translation import trans_real
         trans_real._translations = {}