Django-templates: Why doesn't {% if "string"|length > 10 %} work at all?

I'm recommending not using this but I have combined the get_digit and the length filters before to make this work.

{% if "12345678901234567890"|length|get_digit:"-1" > 20 %} 
    {{ "12345678901234567890"|length }} 
{% endif %}

results in nothing in the template, but:

{% if "12345678901234567890"|length|get_digit:"-1" > 19 %} 
    {{ "12345678901234567890"|length }} 
{% endif %}

results in:

20

being printed.

Yes, filters always return a string.

You can achive the desired functionality by calculating string length in a view and do something like this:

{% if str_length > 10 %}
    {{ str_length }}
{% endif %}

Or create a custom filter for your needs: http://code.djangoproject.com/wiki/BasicComparisonFilters

Edited for typo

I know its late but as per django 2.1 your code will work. Please see below reference https://docs.djangoproject.com/en/2.1/ref/templates/builtins/#filters

{% if messages|length >= 100 %}
   You have lots of messages today!
{% endif %}