Added initial support for loading template engines.

django/template/__init__.py

@@ -1,3 +1,15 @@
+### Multiple Template Engines
+
+from .utils import EngineHandler
+
+
+engines = EngineHandler()
+
+__all__ = ('engines',)
+
+
+### Django Template Language
+
 # Public exceptions
 from .base import (TemplateDoesNotExist, TemplateSyntaxError,           # NOQA
                    VariableDoesNotExist)
@@ -14,4 +26,4 @@ from .base import resolve_variable                                      # NOQA
 from .base import Library                                               # NOQA
 
 
-__all__ = ('Template', 'Context', 'RequestContext')
+__all__ += ('Template', 'Context', 'RequestContext')

django/template/utils.py

@@ -1,9 +1,107 @@
+from collections import Counter, OrderedDict
 import os
 import sys
+import warnings
 
 from django.apps import apps
+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.functional import cached_property
+from django.utils.module_loading import import_string
+
+
+class InvalidTemplateEngineError(ImproperlyConfigured):
+    pass
+
+
+class EngineHandler(object):
+    def __init__(self, templates=None):
+        """
+        templates is an optional list of template engine definitions
+        (structured like settings.TEMPLATES).
+        """
+        self._templates = templates
+        self._engines = {}
+
+    @cached_property
+    def templates(self):
+        if self._templates is None:
+            self._templates = settings.TEMPLATES
+
+        if not self._templates:
+            warnings.warn(
+                "You haven't defined a TEMPLATES setting. You must do so "
+                "before upgrading to Django 2.0. Otherwise Django will be "
+                "unable to load templates.", RemovedInDjango20Warning)
+            self._templates = [
+                {
+                    'BACKEND': 'django.template.backends.django.DjangoTemplates',
+                    'DIRS': settings.TEMPLATE_DIRS,
+                    'OPTIONS': {
+                        'allowed_include_roots': settings.ALLOWED_INCLUDE_ROOTS,
+                        'context_processors': settings.TEMPLATE_CONTEXT_PROCESSORS,
+                        'loaders': settings.TEMPLATE_LOADERS,
+                        'string_if_invalid': settings.TEMPLATE_STRING_IF_INVALID,
+                    },
+                },
+            ]
+
+        templates = OrderedDict()
+        for tpl in self._templates:
+            tpl = tpl.copy()
+            try:
+                # This will raise an exception if 'BACKEND' doesn't exist or
+                # isn't a string containing at least one dot.
+                default_name = tpl['BACKEND'].rsplit('.', 2)[-2]
+            except Exception:
+                invalid_backend = tpl.get('BACKEND', '<not defined>')
+                raise ImproperlyConfigured(
+                    "Invalid BACKEND for a template engine: {}. Check "
+                    "your TEMPLATES setting.".format(invalid_backend))
+
+            tpl.setdefault('NAME', default_name)
+            tpl.setdefault('DIRS', [])
+            tpl.setdefault('APP_DIRS', False)
+            tpl.setdefault('OPTIONS', {})
+
+            templates[tpl['NAME']] = tpl
+
+        counts = Counter(list(templates))
+        duplicates = [alias for alias, count in counts.most_common() if count > 1]
+        if duplicates:
+            raise ImproperlyConfigured(
+                "Template engine aliases aren't unique, duplicates: {}. "
+                "Set a unique NAME for each engine in settings.TEMPLATES."
+                .format(", ".join(duplicates)))
+
+        return templates
+
+    def __getitem__(self, alias):
+        try:
+            return self._engines[alias]
+        except KeyError:
+            try:
+                params = self.templates[alias]
+            except KeyError:
+                raise InvalidTemplateEngineError(
+                    "Could not find config for '{}' "
+                    "in settings.TEMPLATES".format(alias))
+
+            backend = params.pop('BACKEND')
+            engine_cls = import_string(backend)
+            engine = engine_cls(params)
+
+            self._engines[alias] = engine
+            return engine
+
+    def __iter__(self):
+        return iter(self.templates)
+
+    def all(self):
+        return [self[alias] for alias in self]
 
 
 @lru_cache.lru_cache()