Default filter cannot be used with undefined variables

[Mitar]

I wanted to assign a default value from a variable:

{% with heading=heading|default:title|default:"unknown" %}

{% endwith %}

But if variable title is missing, rendering simply fails, without any exception (even in debug mode) but also without setting heading to "unknown". From looking at the source code I understand that this happens because default filter is implemented using simple tags which expect that all filter arguments are defined, but it is unexpected behavior in this particular case.

[Tim Graham]

As you described, I'm not sure it's feasible given the current implementation. Can you propose a patch to fix it?

[Mitar]

Default filter should be probably moved into a special case inside the FilterExpression handling.

[Aymeric Augustin]

That doesn't sound like a very good design :(

[Tim Graham]

I think it will probably be better not to support this. For one, it will force people not to put so much logic in their templates.

[Mitar]

The issue is that there is no error or warning when user does this. Even in debug mode. There should be at least an exception when this happens. Otherwise it is really hard to capture that this is happening.