Fixed #19253 -- Extracted template cache key building logic

Introduced a public function
django.core.cache.utils.make_template_fragment_key
Thanks @chrismedrela for fruitful cooperation.

AUTHORS

@@ -382,6 +382,7 @@ answer newbie questions, and generally made Django that much better:
     Paul McLanahan <paul@mclanahan.net>
     Tobias McNulty <http://www.caktusgroup.com/blog>
     Andrews Medina <andrewsmedina@gmail.com>
+    Christoph Mędrela <chris.medrela@gmail.com>
     Zain Memon
     Christian Metts
     michal@plovarna.cz
@@ -418,6 +419,7 @@ answer newbie questions, and generally made Django that much better:
     Christian Oudard <christian.oudard@gmail.com>
     oggie rob <oz.robharvey@gmail.com>
     oggy <ognjen.maric@gmail.com>
+    Tomek Paczkowski <tomek@hauru.eu>
     Jens Page
     Jay Parlar <parlar@gmail.com>
     Carlos Eduardo de Paula <carlosedp@gmail.com>

django/core/cache/utils.py

+from __future__ import absolute_import, unicode_literals
+
+import hashlib
+from django.utils.encoding import force_bytes
+from django.utils.http import urlquote
+
+TEMPLATE_FRAGMENT_KEY_TEMPLATE = 'template.cache.%s.%s'
+
+
+def make_template_fragment_key(fragment_name, vary_on=None):
+    if vary_on is None:
+        vary_on = ()
+    key = ':'.join([urlquote(var) for var in vary_on])
+    args = hashlib.md5(force_bytes(key))
+    return TEMPLATE_FRAGMENT_KEY_TEMPLATE % (fragment_name, args.hexdigest())

django/templatetags/cache.py

 from __future__ import unicode_literals
 
-import hashlib
+from django.core.cache.utils import make_template_fragment_key
 from django.template import Library, Node, TemplateSyntaxError, VariableDoesNotExist
 from django.core.cache import cache
-from django.utils.encoding import force_bytes
-from django.utils.http import urlquote
 
 register = Library()
 
@@ -24,10 +22,8 @@ class CacheNode(Node):
             expire_time = int(expire_time)
         except (ValueError, TypeError):
             raise TemplateSyntaxError('"cache" tag got a non-integer timeout value: %r' % expire_time)
-        # Build a key for this fragment and all vary-on's.
-        key = ':'.join([urlquote(var.resolve(context)) for var in self.vary_on])
-        args = hashlib.md5(force_bytes(key))
-        cache_key = 'template.cache.%s.%s' % (self.fragment_name, args.hexdigest())
+        vary_on = [var.resolve(context) for var in self.vary_on]
+        cache_key = make_template_fragment_key(self.fragment_name, vary_on)
         value = cache.get(cache_key)
         if value is None:
             value = self.nodelist.render(context)

docs/topics/cache.txt

@@ -639,6 +639,23 @@ equivalent:
 This feature is useful in avoiding repetition in templates. You can set the
 timeout in a variable, in one place, and just reuse that value.
 
+.. function:: django.core.cache.utils.make_template_fragment_key(fragment_name, vary_on=None)
+
+If you want to obtain the cache key used for a cached fragment, you can use
+``make_template_fragment_key``. ``fragment_name`` is the same as second argument
+to the ``cache`` template tag; ``vary_on`` is a list of all additional arguments
+passed to the tag. This function can be useful for invalidating or overwriting
+a cached item, for example:
+
+.. code-block:: python
+
+    >>> from django.core.cache import cache
+    >>> from django.core.cache.utils import make_template_fragment_key
+    # cache key for {% cache 500 sidebar username %}
+    >>> key = make_template_fragment_key('sidebar', [username])
+    >>> cache.delete(key) # invalidates cached template fragment
+
+
 The low-level cache API
 =======================
 

tests/regressiontests/cache/tests.py

@@ -20,6 +20,7 @@ from django.core.cache import get_cache
 from django.core.cache.backends.base import (CacheKeyWarning,
     InvalidCacheBackendError)
 from django.db import router, transaction
+from django.core.cache.utils import make_template_fragment_key
 from django.http import (HttpResponse, HttpRequest, StreamingHttpResponse,
     QueryDict)
 from django.middleware.cache import (FetchFromCacheMiddleware,
@@ -1809,3 +1810,25 @@ class TestEtagWithAdmin(TestCase):
             response = self.client.get('/test_admin/admin/')
             self.assertEqual(response.status_code, 200)
             self.assertTrue(response.has_header('ETag'))
+
+
+class TestMakeTemplateFragmentKey(TestCase):
+    def test_without_vary_on(self):
+        key = make_template_fragment_key('a.fragment')
+        self.assertEqual(key, 'template.cache.a.fragment.d41d8cd98f00b204e9800998ecf8427e')
+
+    def test_with_one_vary_on(self):
+        key = make_template_fragment_key('foo', ['abc'])
+        self.assertEqual(key,
+            'template.cache.foo.900150983cd24fb0d6963f7d28e17f72')
+
+    def test_with_many_vary_on(self):
+        key = make_template_fragment_key('bar', ['abc', 'def'])
+        self.assertEqual(key,
+            'template.cache.bar.4b35f12ab03cec09beec4c21b2d2fa88')
+
+    def test_proper_escaping(self):
+        key = make_template_fragment_key('spam', ['abc:def%'])
+        self.assertEqual(key,
+            'template.cache.spam.f27688177baec990cdf3fbd9d9c3f469')
+