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.

8 files changed, 102 insertions, 93 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
diff --git a/tests/template_tests/test_loaders.py b/tests/template_tests/test_loaders.py
index e83b8060b1..c3c086f381 100644
--- a/tests/template_tests/test_loaders.py
+++ b/tests/template_tests/test_loaders.py
@@ -22,6 +22,7 @@ except ImportError:
 
 from django.template import TemplateDoesNotExist, Context
 from django.template.loaders.eggs import Loader as EggLoader
+from django.template.loaders.utils import find_template_loader
 from django.template import loader
 from django.test import TestCase, override_settings
 from django.utils import six
@@ -127,7 +128,7 @@ class CachedLoader(TestCase):
 
     def test_missing_template_is_cached(self):
         "#19949 -- Check that the missing template is cached."
-        template_loader = loader.find_template_loader(settings.TEMPLATE_LOADERS[0])
+        template_loader = find_template_loader(settings.TEMPLATE_LOADERS[0])
         # Empty cache, which may be filled from previous tests.
         template_loader.reset()
         # Check that 'missing.html' isn't already in cache before 'missing.html' is loaded
diff --git a/tests/template_tests/tests.py b/tests/template_tests/tests.py
index 334dd31f7b..b8097fbde1 100644
--- a/tests/template_tests/tests.py
+++ b/tests/template_tests/tests.py
@@ -14,7 +14,8 @@ from django.contrib.auth.models import Group
 from django.core import urlresolvers
 from django.template import (base as template_base, loader, Context,
     RequestContext, Template, TemplateSyntaxError)
-from django.template.loaders import app_directories, filesystem, cached
+from django.template.loaders import app_directories, filesystem
+from django.template.loaders.utils import get_template_loaders
 from django.test import RequestFactory, TestCase
 from django.test.utils import override_settings, extend_sys_path
 from django.utils.deprecation import RemovedInDjango19Warning, RemovedInDjango20Warning
@@ -218,21 +219,27 @@ class TemplateLoaderTests(TestCase):
     @override_settings(TEMPLATE_DEBUG=True)
     def test_loader_debug_origin(self):
         # We rely on the fact that runtests.py sets up TEMPLATE_DIRS to
-        # point to a directory containing a login.html file. Also that
-        # the file system and app directories loaders both inherit the
-        # load_template method from the base Loader class, so we only need
-        # to test one of them.
+        # point to a directory containing a login.html file.
         load_name = 'login.html'
+
+        # We also rely on the fact the file system and app directories loaders
+        # both inherit the load_template method from the base Loader class, so
+        # we only need to test one of them.
         template = loader.get_template(load_name)
         template_name = template.nodelist[0].source[0].name
         self.assertTrue(template_name.endswith(load_name),
             'Template loaded by filesystem loader has incorrect name for debug page: %s' % template_name)
 
-        # Also test the cached loader, since it overrides load_template
-        cache_loader = cached.Loader(('',))
-        cache_loader._cached_loaders = loader.template_source_loaders
-        loader.template_source_loaders = (cache_loader,)
+    @override_settings(TEMPLATE_LOADERS=[
+        ('django.template.loaders.cached.Loader',
+            ['django.template.loaders.filesystem.Loader']),
+    ])
+    @override_settings(TEMPLATE_DEBUG=True)
+    def test_cached_loader_debug_origin(self):
+        # Same comment as in test_loader_debug_origin.
+        load_name = 'login.html'
 
+        # Test the cached loader separately since it overrides load_template.
         template = loader.get_template(load_name)
         template_name = template.nodelist[0].source[0].name
         self.assertTrue(template_name.endswith(load_name),
@@ -243,15 +250,15 @@ class TemplateLoaderTests(TestCase):
         self.assertTrue(template_name.endswith(load_name),
             'Cached template loaded through cached loader has incorrect name for debug page: %s' % template_name)
 
+    @override_settings(TEMPLATE_DEBUG=True)
     def test_loader_origin(self):
-        with self.settings(TEMPLATE_DEBUG=True):
-            template = loader.get_template('login.html')
-            self.assertEqual(template.origin.loadname, 'login.html')
+        template = loader.get_template('login.html')
+        self.assertEqual(template.origin.loadname, 'login.html')
 
+    @override_settings(TEMPLATE_DEBUG=True)
     def test_string_origin(self):
-        with self.settings(TEMPLATE_DEBUG=True):
-            template = Template('string template')
-            self.assertEqual(template.origin.source, 'string template')
+        template = Template('string template')
+        self.assertEqual(template.origin.source, 'string template')
 
     def test_debug_false_origin(self):
         template = loader.get_template('login.html')
@@ -613,7 +620,9 @@ class TemplateTests(TestCase):
                                 if output != result:
                                     failures.append("Template test (Cached='%s', TEMPLATE_STRING_IF_INVALID='%s', TEMPLATE_DEBUG=%s): %s -- FAILED. Expected %r, got %r" % (is_cached, invalid_str, template_debug, name, result, output))
 
-                    loader.template_source_loaders[0].reset()
+                    # This relies on get_template_loaders() memoizing its
+                    # result. All callers get the same iterable of loaders.
+                    get_template_loaders()[0].reset()
 
                 if template_base.invalid_var_format_string:
                     expected_invalid_str = 'INVALID'
diff --git a/tests/test_client_regress/tests.py b/tests/test_client_regress/tests.py
index 9ceba4992f..f7dcae15cf 100644
--- a/tests/test_client_regress/tests.py
+++ b/tests/test_client_regress/tests.py
@@ -8,8 +8,7 @@ import os
 import itertools
 
 from django.core.urlresolvers import reverse, NoReverseMatch
-from django.template import (TemplateSyntaxError,
-    Context, Template, loader)
+from django.template import TemplateSyntaxError, Context, Template
 import django.template.context
 from django.test import Client, TestCase, override_settings
 from django.test.client import encode_file, RequestFactory
@@ -902,13 +901,6 @@ class ExceptionTests(TestCase):
 @override_settings(ROOT_URLCONF='test_client_regress.urls')
 class TemplateExceptionTests(TestCase):
 
-    def setUp(self):
-        # Reset the loaders so they don't try to render cached templates.
-        if loader.template_source_loaders is not None:
-            for template_loader in loader.template_source_loaders:
-                if hasattr(template_loader, 'reset'):
-                    template_loader.reset()
-
     @override_settings(
         TEMPLATE_DIRS=(os.path.join(os.path.dirname(upath(__file__)), 'bad_templates'),)
     )
@@ -916,9 +908,10 @@ class TemplateExceptionTests(TestCase):
         "Errors found when rendering 404 error templates are re-raised"
         try:
             self.client.get("/no_such_view/")
-            self.fail("Should get error about syntax error in template")
         except TemplateSyntaxError:
             pass
+        else:
+            self.fail("Should get error about syntax error in template")
 
 
 # We need two different tests to check URLconf substitution -  one to check