Prevent nonexistent template filter arguments from raising VariableDoesNotExist

[Vlastimil Zíma]

I'd like to fix the error reported in #13167. The main bug

from django.template import Context, Template
Template('{{ foo|default:notreal }}').render(Context({'foo': ''}))

still raises VariableDoesNotExist exception, which is completely unexpected from templates, as noted in closing comment ticket:13167#comment:20. I took me more than an hour before I found out how did it make the server error I investigated.

I know the original ticket was closed as wontfix, but I understood it was due to the effect the proposed change would have on if tag. But there's another way - capture the exception in VariableNode the similar way UnicodeDecodeError is silenced. If I understood the code correctly, that would solve the problem for all template filters, but left template tags intact.

[Tim Graham]

I understand the "template errors don't raise exceptions" philosophy, however, I think changing the current behavior would be more error prone (e.g. typos in variable names go more easily undetected). I've started a ​django-developers thread to get other opinions.

[Tim Graham]

Alejandro, I don't think there's consensus to make this change (hence the ticket's Someday/Maybe status). Did you read the mailing list discussion linked from in comment 1?