Encapsulated TEMPLATE_CONTEXT_PROCESSORS in Engine.

Since RequestContext doesn't know its Engine until it's passed to
Template.render() -- and cannot without breaking a widely used public
API -- an elaborate hack is required to apply context processors.

django/template/context.py

 from copy import copy
 
-from django.conf import settings
-from django.utils import lru_cache
-from django.utils.module_loading import import_string
 
 # Hard-coded processor for easier use of CSRF protection.
 _builtin_context_processors = ('django.core.context_processors.csrf',)
@@ -172,13 +169,6 @@ class RenderContext(BaseContext):
         return self.dicts[-1][key]
 
 
-@lru_cache.lru_cache()
-def get_standard_processors():
-    context_processors = _builtin_context_processors
-    context_processors += tuple(settings.TEMPLATE_CONTEXT_PROCESSORS)
-    return tuple(import_string(path) for path in context_processors)
-
-
 class RequestContext(Context):
     """
     This subclass of template.Context automatically populates itself using
@@ -190,11 +180,33 @@ class RequestContext(Context):
             use_l10n=None, use_tz=None, engine=None):
         Context.__init__(self, dict_, current_app=current_app,
                 use_l10n=use_l10n, use_tz=use_tz, engine=engine)
-        if processors is None:
-            processors = ()
-        else:
-            processors = tuple(processors)
-        updates = dict()
-        for processor in get_standard_processors() + processors:
-            updates.update(processor(request))
-        self.update(updates)
+        self._request = request
+        self._processors = () if processors is None else tuple(processors)
+        self._processors_index = len(self.dicts)
+        self.update({})         # placeholder for context processors output
+        self.engine = engine    # re-run the setter in case engine is not None
+
+    @property
+    def engine(self):
+        return self._engine
+
+    @engine.setter
+    def engine(self, engine):
+        self._engine = engine
+        if hasattr(self, '_processors_index'):
+            if engine is None:
+                # Unset context processors.
+                self.dicts[self._processors_index] = {}
+            else:
+                # Set context processors for this engine.
+                updates = {}
+                for processor in engine.template_context_processors + self._processors:
+                    updates.update(processor(self._request))
+                self.dicts[self._processors_index] = updates
+
+    def new(self, values=None):
+        new_context = super(RequestContext, self).new(values)
+        # This is for backwards-compatibility: RequestContexts created via
+        # Context.new don't include values from context processors.
+        del new_context._processors_index
+        return new_context

django/template/engine.py

@@ -9,6 +9,7 @@ from django.utils.functional import cached_property
 from django.utils.module_loading import import_string
 
 from .base import Context, Lexer, Parser, Template, TemplateDoesNotExist
+from .context import _builtin_context_processors
 
 
 _dirs_undefined = object()
@@ -58,6 +59,12 @@ class Engine(object):
             file_charset=settings.FILE_CHARSET,
         )
 
+    @cached_property
+    def template_context_processors(self):
+        context_processors = _builtin_context_processors
+        context_processors += tuple(self.context_processors)
+        return tuple(import_string(path) for path in context_processors)
+
     @cached_property
     def template_loaders(self):
         return self.get_template_loaders(self.loaders)

django/test/signals.py

@@ -91,13 +91,6 @@ def reset_default_template_engine(**kwargs):
         Engine.get_default.cache_clear()
 
 
-@receiver(setting_changed)
-def clear_context_processors_cache(**kwargs):
-    if kwargs['setting'] == 'TEMPLATE_CONTEXT_PROCESSORS':
-        from django.template.context import get_standard_processors
-        get_standard_processors.cache_clear()
-
-
 @receiver(setting_changed)
 def clear_serializers_cache(**kwargs):
     if kwargs['setting'] == 'SERIALIZATION_MODULES':