VariableDoesNotExist can lead to stack overflows unusually easily

[Ryan O’Hara]

If a form is overridden to render a template (​django-floppyforms does this):

class ExampleForm(forms.Form):
    def as_table(self):
        return render(None, 'example.html', {'form': self})

and the template refers to a nonexistent attribute on the form:

{{ form.boom }}

printing the VariableDoesNotExist will cause it to attempt to stringify the form for context again.

Tentative patch changes it to always repr(), which seems appropriate and has the advantage of showing dict context like this:

Failed lookup for key [author] in {'section': '"News"'}

instead of like this:

Failed lookup for key [author] in '{\'section\': \'"News"\'}'

[Ryan O’Hara]

​https://github.com/django/django/pull/7957

[Thomas Hauk]

Also the cause of an exception death spiral in DRF, c.f.: ​https://github.com/tomchristie/django-rest-framework/issues/3736

Would love to see this patch integrated into Django!

[Tim Graham <timograham@…>]

In 7c4289d:

Fixed #27783 -- Switched VariableDoesNotExist.str() to repr() context.

Using str() and then repr'ing the result looks strange and can lead
to recursive rendering of forms.