Opened 7 years ago

Closed 7 years ago

#22767 closed Bug (duplicate)

Unexpected value for an unset variable with length filter in templates

Reported by: György Kiss Owned by: nobody
Component: Template system Version: 1.6
Severity: Normal Keywords:
Cc: gregchapple1@… Triage Stage: Unreviewed
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


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.

Change History (4)

comment:1 Changed 7 years ago by 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 %}
    {% else %}
       no groups :(
    {% endif %}
{% else %}
    groups not defined
{% end %}
Version 0, edited 7 years ago by Greg Chapple (next)

comment:2 Changed 7 years ago by 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?

comment:3 Changed 7 years ago by Greg Chapple

Cc: gregchapple1@… added

comment:4 Changed 7 years ago by Tim Graham

Resolution: duplicate
Status: newclosed

Duplicate of #18400.

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