[1.9.x] Fixed #26018 -- Prevented unecessary get_form() call in FormMixin.get_context_data().

Changed "dict.setdefault" to "if x in dict" pattern so that get_form() would not
be called unnecessarily, specifically in the case where FormMixin.form_invalid()
calls get_context_data() with the current form.

Backport of e429c5186ceed81c4627165518e0c70c58e69595 from master

2 files changed, 7 insertions, 0 deletions

diff --git a/tests/generic_views/test_edit.py b/tests/generic_views/test_edit.py
index e0730b1118..ddbdcadf35 100644
--- a/tests/generic_views/test_edit.py
+++ b/tests/generic_views/test_edit.py
@@ -260,6 +260,7 @@ class UpdateViewTests(TestCase):
         self.assertEqual(res.context['object'], Author.objects.get(pk=a.pk))
         self.assertEqual(res.context['author'], Author.objects.get(pk=a.pk))
         self.assertTemplateUsed(res, 'generic_views/author_form.html')
+        self.assertEqual(res.context['view'].get_form_called_count, 1)
 
         # Modification with both POST and PUT (browser compatible)
         res = self.client.post('/edit/author/%d/update/' % a.pk,
@@ -279,6 +280,7 @@ class UpdateViewTests(TestCase):
         self.assertTemplateUsed(res, 'generic_views/author_form.html')
         self.assertEqual(len(res.context['form'].errors), 1)
         self.assertQuerysetEqual(Author.objects.all(), ['<Author: Randall Munroe>'])
+        self.assertEqual(res.context['view'].get_form_called_count, 1)
 
     def test_update_with_object_url(self):
         a = Artist.objects.create(name='Rene Magritte')
diff --git a/tests/generic_views/views.py b/tests/generic_views/views.py
index b7df1e410b..3ab5aca468 100644
--- a/tests/generic_views/views.py
+++ b/tests/generic_views/views.py
@@ -147,10 +147,15 @@ class NaiveAuthorUpdate(generic.UpdateView):
 
 
 class AuthorUpdate(generic.UpdateView):
+    get_form_called_count = 0  # Used to ensure get_form() is called once.
     model = Author
     success_url = '/list/authors/'
     fields = '__all__'
 
+    def get_form(self, *args, **kwargs):
+        self.get_form_called_count += 1
+        return super(AuthorUpdate, self).get_form(*args, **kwargs)
+
 
 class OneAuthorUpdate(generic.UpdateView):
     success_url = '/list/authors/'