Fixed #23656 -- Made FormMixin.get_form's form_class argument optional.

Thanks Tim Graham for the review.

tests/generic_views/test_edit.py

@@ -1,12 +1,14 @@
 from __future__ import unicode_literals
 
 from unittest import expectedFailure
+import warnings
 
 from django.core.exceptions import ImproperlyConfigured
 from django.core.urlresolvers import reverse
 from django import forms
 from django.test import TestCase, override_settings
 from django.test.client import RequestFactory
+from django.utils.deprecation import RemovedInDjango20Warning
 from django.views.generic.base import View
 from django.views.generic.edit import FormMixin, ModelFormMixin, CreateView
 
@@ -40,6 +42,43 @@ class FormMixinTests(TestCase):
         set_kwargs = set_mixin.get_form_kwargs()
         self.assertEqual(test_string, set_kwargs.get('prefix'))
 
+    def test_get_form(self):
+        class TestFormMixin(FormMixin):
+            request = RequestFactory().get('/')
+
+        self.assertIsInstance(
+            TestFormMixin().get_form(forms.Form), forms.Form,
+            'get_form() should use provided form class.'
+        )
+
+        class FormClassTestFormMixin(TestFormMixin):
+            form_class = forms.Form
+
+        self.assertIsInstance(
+            FormClassTestFormMixin().get_form(), forms.Form,
+            'get_form() should fallback to get_form_class() if none is provided.'
+        )
+
+    def test_get_form_missing_form_class_default_value(self):
+        with warnings.catch_warnings(record=True) as w:
+            class MissingDefaultValue(FormMixin):
+                request = RequestFactory().get('/')
+                form_class = forms.Form
+
+                def get_form(self, form_class):
+                    return form_class(**self.get_form_kwargs())
+        self.assertEqual(len(w), 1)
+        self.assertEqual(w[0].category, RemovedInDjango20Warning)
+        self.assertEqual(
+            str(w[0].message),
+            '`generic_views.test_edit.MissingDefaultValue.get_form` method '
+            'must define a default value for its `form_class` argument.'
+        )
+
+        self.assertIsInstance(
+            MissingDefaultValue().get_form(), forms.Form,
+        )
+
 
 @override_settings(ROOT_URLCONF='generic_views.urls')
 class BasicFormTests(TestCase):