Fixed #20404 -- Added a keys() method to ContextList.

It's useful to be able to list all the (flattened) keys of a
ContextList, to help you figure out why the variable that's supposed
to be there is not.

No .values() or .items() added as the definition for those aren't clear.

The patch is Chris Wilson's patch from pull request 1065 with some
modifications by committer.

django/test/utils.py

@@ -60,6 +60,16 @@ class ContextList(list):
             return False
         return True
 
+    def keys(self):
+        """
+        Flattened keys of subcontexts.
+        """
+        keys = set()
+        for subcontext in self:
+            for dict in subcontext:
+                keys |= set(dict.keys())
+        return keys
+
 
 def instrumented_test_render(self, context):
     """

tests/test_client_regress/tests.py

@@ -6,9 +6,8 @@ from __future__ import unicode_literals
 
 import os
 
-from django.conf import settings
 from django.core.urlresolvers import reverse
-from django.template import (TemplateDoesNotExist, TemplateSyntaxError,
+from django.template import (TemplateSyntaxError,
     Context, Template, loader)
 import django.template.context
 from django.test import Client, TestCase
@@ -897,6 +896,21 @@ class ContextTests(TestCase):
         except KeyError as e:
             self.assertEqual(e.args[0], 'does-not-exist')
 
+    def test_contextlist_keys(self):
+        c1 = Context()
+        c1.update({'hello': 'world', 'goodbye': 'john'})
+        c1.update({'hello': 'dolly', 'dolly': 'parton'})
+        c2 = Context()
+        c2.update({'goodbye': 'world', 'python': 'rocks'})
+        c2.update({'goodbye': 'dolly'})
+
+        l = ContextList([c1, c2])
+        # None, True and False are builtins of BaseContext, and present
+        # in every Context without needing to be added.
+        self.assertEqual(set(['None', 'True', 'False', 'hello', 'goodbye',
+                              'python', 'dolly']),
+                         l.keys())
+
     def test_15368(self):
         # Need to insert a context processor that assumes certain things about
         # the request instance. This triggers a bug caused by some ways of