add template filter doesn't work with mixed types

[Daniel]

{% with identifier="#id"|add:forloop.counter %}
{{  identifier }}
{% endwith %}

print empty value

[Tim Graham]

The issue is that the add filter doesn't support mixed types (string and integer in this case). It might be reasonable to add such support.

[Konrad Świat]

​https://docs.djangoproject.com/en/1.8/ref/templates/builtins/#add

This filter will first try to coerce both values to integers. If this fails, it’ll attempt to add the values together anyway. This will work on some data types (strings, list, etc.) and fail on others. If it fails, the result will be an empty string.

References: #11687, #16676

The question is, how the add tag should behave. If adding fails, do we expect the result to be concatenated string? Maybe we need separate tag to do that?

[Tim Graham]

I guess it's a "won't fix" based on the discussion in #16676 then. I don't think Django needs to add a new filter for this case.

[Chad Stovern]

This seems to not be a bug. I did some simple tests to confirm the behavior is consistent with the documentation.

The add filter correctly concats strings.
It also correctly coerces a string to an int adding allowing it to be added to a subsequent int.
Two ints are also correctly added.
Lastly a string like my name fails to be coerced to an int and when added to an int returns an empty value.

Once again, this is consistent with the docs. Thanks.