[1.9.x] Fixed #25548 -- Prevented FormView.form_invalid() from discarding its form argument.

Backport of e171a83b15 from master

django/views/generic/edit.py

@@ -112,7 +112,7 @@ class FormMixin(six.with_metaclass(FormMixinBase, ContextMixin)):
         If the form is invalid, re-render the context data with the
         data-filled form and errors.
         """
-        return self.render_to_response(self.get_context_data())
+        return self.render_to_response(self.get_context_data(form=form))
 
     def get_context_data(self, **kwargs):
         """

docs/releases/1.9.1.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed ``BaseCache.get_or_set()`` with the ``DummyCache`` backend
   (:ticket:`25840`).
+
+* Fixed a regression in ``FormMixin`` causing forms to be validated twice
+  (:ticket:`25548`).

tests/generic_views/test_edit.py

@@ -98,6 +98,13 @@ class BasicFormTests(TestCase):
         res = self.client.post('/contact/', {'name': "Me", 'message': "Hello"})
         self.assertRedirects(res, '/list/authors/')
 
+    def test_late_form_validation(self):
+        """
+        A form can be marked invalid in the form_valid() method (#25548).
+        """
+        res = self.client.post('/late-validation/', {'name': "Me", 'message': "Hello"})
+        self.assertFalse(res.context['form'].is_valid())
+
 
 class ModelFormMixinTests(SimpleTestCase):
     def test_get_form(self):

tests/generic_views/urls.py

@@ -65,6 +65,8 @@ urlpatterns = [
     # FormView
     url(r'^contact/$',
         views.ContactView.as_view()),
+    url(r'^late-validation/$',
+        views.LateValidationView.as_view()),
 
     # Create/UpdateView
     url(r'^edit/artists/create/$',

tests/generic_views/views.py

@@ -307,3 +307,13 @@ class NonModelDetail(generic.DetailView):
 class ObjectDoesNotExistDetail(generic.DetailView):
     def get_queryset(self):
         return Book.does_not_exist.all()
+
+
+class LateValidationView(generic.FormView):
+    form_class = ContactForm
+    success_url = reverse_lazy('authors_list')
+    template_name = 'generic_views/form.html'
+
+    def form_valid(self, form):
+        form.add_error(None, 'There is an error')
+        return self.form_invalid(form)