Refactored getting the list of template loaders.

This provides the opportunity to move utility functions specific to the
Django Template Language outside of django.template.loader.

5 files changed, 72 insertions, 66 deletions

diff --git a/django/template/loader.py b/django/template/loader.py
index c62c383322..093d7d172b 100644
--- a/django/template/loader.py
+++ b/django/template/loader.py
@@ -1,13 +1,9 @@
 import warnings
 
-from django.core.exceptions import ImproperlyConfigured
-from django.template.base import Origin, Template, Context, TemplateDoesNotExist
 from django.conf import settings
+from django.template.base import Origin, Template, Context, TemplateDoesNotExist
+from django.template.loaders.utils import get_template_loaders
 from django.utils.deprecation import RemovedInDjango20Warning
-from django.utils.module_loading import import_string
-from django.utils import six
-
-template_source_loaders = None
 
 
 class LoaderOrigin(Origin):
@@ -26,58 +22,8 @@ def make_origin(display_name, loader, name, dirs):
         return None
 
 
-def find_template_loader(loader):
-    if isinstance(loader, (tuple, list)):
-        loader, args = loader[0], loader[1:]
-    else:
-        args = []
-    if isinstance(loader, six.string_types):
-        TemplateLoader = import_string(loader)
-
-        if hasattr(TemplateLoader, 'load_template_source'):
-            func = TemplateLoader(*args)
-        else:
-            warnings.warn(
-                "Function-based template loaders are deprecated. Please use "
-                "class-based template loaders instead. Inherit base.Loader "
-                "and provide a load_template_source() method.",
-                RemovedInDjango20Warning, stacklevel=2)
-
-            # Try loading module the old way - string is full path to callable
-            if args:
-                raise ImproperlyConfigured(
-                    "Error importing template source loader %s - can't pass "
-                    "arguments to function-based loader." % loader
-                )
-            func = TemplateLoader
-
-        if not func.is_usable:
-            import warnings
-            warnings.warn(
-                "Your TEMPLATE_LOADERS setting includes %r, but your Python "
-                "installation doesn't support that type of template loading. "
-                "Consider removing that line from TEMPLATE_LOADERS." % loader
-            )
-            return None
-        else:
-            return func
-    else:
-        raise ImproperlyConfigured('Loader does not define a "load_template" callable template source loader')
-
-
 def find_template(name, dirs=None):
-    # Calculate template_source_loaders the first time the function is executed
-    # because putting this logic in the module-level namespace may cause
-    # circular import errors. See Django ticket #1292.
-    global template_source_loaders
-    if template_source_loaders is None:
-        loaders = []
-        for loader_name in settings.TEMPLATE_LOADERS:
-            loader = find_template_loader(loader_name)
-            if loader is not None:
-                loaders.append(loader)
-        template_source_loaders = tuple(loaders)
-    for loader in template_source_loaders:
+    for loader in get_template_loaders():
         try:
             source, display_name = loader(name, dirs)
             return (source, make_origin(display_name, loader, name, dirs))
diff --git a/django/template/loaders/cached.py b/django/template/loaders/cached.py
index e217efc93f..180ed9900d 100644
--- a/django/template/loaders/cached.py
+++ b/django/template/loaders/cached.py
@@ -5,7 +5,8 @@ to load templates from them in order, caching the result.
 
 import hashlib
 from django.template.base import TemplateDoesNotExist
-from django.template.loader import get_template_from_string, find_template_loader, make_origin
+from django.template.loader import get_template_from_string, make_origin
+from django.template.loaders.utils import find_template_loader
 from django.utils.encoding import force_bytes
 
 from .base import Loader as BaseLoader
diff --git a/django/template/loaders/utils.py b/django/template/loaders/utils.py
new file mode 100644
index 0000000000..4850c9f1aa
--- /dev/null
+++ b/django/template/loaders/utils.py
@@ -0,0 +1,57 @@
+import warnings
+
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+from django.utils import lru_cache
+from django.utils import six
+from django.utils.deprecation import RemovedInDjango20Warning
+from django.utils.module_loading import import_string
+
+
+@lru_cache.lru_cache()
+def get_template_loaders():
+    loaders = []
+    for loader_name in settings.TEMPLATE_LOADERS:
+        loader = find_template_loader(loader_name)
+        if loader is not None:
+            loaders.append(loader)
+    # Immutable return value because it will be cached and shared by callers.
+    return tuple(loaders)
+
+
+def find_template_loader(loader):
+    if isinstance(loader, (tuple, list)):
+        loader, args = loader[0], loader[1:]
+    else:
+        args = []
+    if isinstance(loader, six.string_types):
+        TemplateLoader = import_string(loader)
+
+        if hasattr(TemplateLoader, 'load_template_source'):
+            func = TemplateLoader(*args)
+        else:
+            warnings.warn(
+                "Function-based template loaders are deprecated. "
+                "Please use class-based template loaders instead. "
+                "Inherit django.template.loaders.base.Loader "
+                "and provide a load_template_source() method.",
+                RemovedInDjango20Warning, stacklevel=2)
+
+            # Try loading module the old way - string is full path to callable
+            if args:
+                raise ImproperlyConfigured(
+                    "Error importing template source loader %s - can't pass "
+                    "arguments to function-based loader." % loader)
+            func = TemplateLoader
+
+        if not func.is_usable:
+            warnings.warn(
+                "Your TEMPLATE_LOADERS setting includes %r, but your Python "
+                "installation doesn't support that type of template loading. "
+                "Consider removing that line from TEMPLATE_LOADERS." % loader)
+            return None
+        else:
+            return func
+    else:
+        raise ImproperlyConfigured(
+            "Invalid value in TEMPLATE_LOADERS: %r" % loader)
diff --git a/django/test/signals.py b/django/test/signals.py
index e080d976ee..f065e155b8 100644
--- a/django/test/signals.py
+++ b/django/test/signals.py
@@ -87,8 +87,8 @@ def clear_context_processors_cache(**kwargs):
 @receiver(setting_changed)
 def clear_template_loaders_cache(**kwargs):
     if kwargs['setting'] == 'TEMPLATE_LOADERS':
-        from django.template import loader
-        loader.template_source_loaders = None
+        from django.template.loaders.utils import get_template_loaders
+        get_template_loaders.cache_clear()
 
 
 @receiver(setting_changed)
diff --git a/django/views/debug.py b/django/views/debug.py
index 95264afbd5..59136ce90c 100644
--- a/django/views/debug.py
+++ b/django/views/debug.py
@@ -12,6 +12,7 @@ from django.http import (HttpResponse, HttpResponseNotFound, HttpRequest,
     build_request_repr)
 from django.template import Template, Context, TemplateDoesNotExist
 from django.template.defaultfilters import force_escape, pprint
+from django.template.loaders.utils import get_template_loaders
 from django.utils.datastructures import MultiValueDict
 from django.utils.html import escape
 from django.utils.encoding import force_bytes, smart_text
@@ -279,14 +280,15 @@ class ExceptionReporter(object):
         """Return a dictionary containing traceback information."""
 
         if self.exc_type and issubclass(self.exc_type, TemplateDoesNotExist):
-            from django.template.loader import template_source_loaders
             self.template_does_not_exist = True
             self.loader_debug_info = []
-            # If the template_source_loaders haven't been populated yet, you need
-            # to provide an empty list for this for loop to not fail.
-            if template_source_loaders is None:
-                template_source_loaders = []
-            for loader in template_source_loaders:
+            # If Django fails in get_template_loaders, provide an empty list
+            # for the following loop to not fail.
+            try:
+                template_loaders = get_template_loaders()
+            except Exception:
+                template_loaders = []
+            for loader in template_loaders:
                 try:
                     source_list_func = loader.get_template_sources
                     # NOTE: This assumes exc_value is the name of the template that