FormMixin.get_context_data calls get_form improperly after form_invalid

[Chris Cogdon]

In the fix for ticket #25548, the validated (and failed) form is now passed in as a parameter to get_context_data. In that code, self.get_form() is called unnecessarily, which will likely cause a performance and side-effect penalty equivalent or worse to validating the form twice.

Solution is to use the "if x not in d" pattern instead of setdefault.

I'll create a pull request.

[Chris Cogdon]

Pull request: ​https://github.com/django/django/pull/5900

Release notes for 1.9.1 updated. Docs compile without error. Test case added. All tests pass under Sqlite.

[Chris Cogdon]

Note: I added a call-counter stub to the AuthorUpdate view. Ideally I'd use the "mock" library to do this kind of thing, as that would also let me check the call-count after form_valid, rather than only being able to test if res.context['view'] was present.

[Tim Graham]

Thanks, I'll merge the release note with the one for #25548 since it seems this issue isn't present in 1.9 but rather introduced in the fix for that ticket as you mentioned.

By the way, we have automated checks on the pull request for docs building without error and tests passing, so there's no need to mention all that here.

[Tim Graham <timograham@…>]

In e429c51:

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.

[Tim Graham <timograham@…>]

In 8202ce45:

[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