Unexpected value for an unset variable with length filter in templates

[György Kiss]

Not sure if this is a bug or I'm just missing something, but I expected this to be False anyway:
{% if groups|length > 0 %}

This will be True in Django 1.6.5 if groups is an unset variable. The TEMPLATE_STRING_IF_INVALID setting has the default value (empty string).

I know it's not idiomatic code and I fixed it:
{% if groups %}
(which works as expected), but other newbies like me can have a hard time figuring out what's wrong.

I checked {% if None|length > 0 %} and it is also True, which is clearly not expected.

[Greg Chapple]

I agree that this is somewhat unexpected, though I don't know if it's a bug. If you were to do the same in Python, and tried to get the length of something which is None, you would get a TypeError. To get around that you would either use a try/except block, or first check if the variable is not None. I think it's reasonable to do the same in your templates. ie:

{% if groups %}
    {% if groups|length > 0 %}
       groups!
    {% else %}
       no groups :(
    {% endif %}
{% else %}
    groups not defined
{% end %}

[Greg Chapple]

Saying that, I would still expect the length filter to return 0 unless the length is indeed something other than that. From taking a quick look, it seems to return an empty string if the value given does not support len(). I changed this to instead return 0 and it does indeed fix the issue, though it makes a ton of tests fail. Perhaps someone with a little more experience with filters can provide some insight as to why it currently returns a string?

[Tim Graham]

Duplicate of #18400.