Formset non_form_errors returns ErrorList() if is_valid is not called

[vbmendes]

If Formset.non_form_errors() is called before Formset.is_valid(), it will return an empty ErrorList.
Let's pretend we have two formsets in one view and we make this test: if formset1.is_valid() and formset2.is_valid(). If formset1 is not valid, formset2's is_valid() will not be called. It means that the user will have to fix all the formset1's validation erros before knowing that formset2 also has errors. It's annoying for the user.

I wrote a little patch to fix this issue. Just changing a little the Formset.non_form_errors() function.

[vbmendes]

First patch

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[senko]

The patch "formset_non_form_errors.diff" doesn't do enough, because if non_form_errors() is called multiple times for any reasons, and there weren't any non-form errors, it will attempt the validation multiple times.

The fix I got in a pull request at ​https://github.com/django/django/pull/1094 is slightly different: full_clean() always sets the _non_form_errors (either to an empty error list, or to the actual errors if there are any), and non_field_errors() just check whether it needs to run full_clean() beforehand.

[Tome Cvitan]

I have reviewed the solution, looks good as far as I can see.

[Senko Rasic <senko.rasic@…>]

In 493aca453aa94f160764d9a89c8043f7c9a67a78:

Fixed #11160 - Ensure full_clean is called from non_form_errors

Updated FormSet.non_form_errors() to ensure full_clean() has
been called before returning the errors.

[Andrew Godwin <andrew@…>]

In 04e0fc029f6868221dbba850e5a8830c8d11c42c:

Merge pull request #1094 from senko/ticket_11160

Fixed #11160: Formset non_form_errors returns ErrorList() if is_valid is not called