crafting (and) JavaScript

What is -%} for, in a Jinja Template? or The Magic Dash in a Jinja Template Block

What the heck does -%} mean in a Jinja template?
TL;DR: It removes whitespaces inside a block. The offical docs say it "strip[s] whitespace in templates [...] add a minus sign (-) to the start or end of a block", so {%- at the end of the block works too, but you better play with around with it to really understand it the dash is quite flexible.

Example with all Whitespaces Removed - A lot of Dashes Used

Dear {{- name -}}

{%- if not asleep -%}    
{%- endif -%}

The end

Notice the six dashes, not only in blocks {%- and -%} but also when rendering variables {{- and -}}. (I did not find this in the docs though). The above template renders out the following result:


Example with No Whitespaces Removed - no dashes used

The same template, just all dashes removed looking like this:

Dear {{ name }}

{% if not asleep %}    
{% endif %}

The end

renders a lot spaces and newlines:




FYI: I tried all of the above in jinja.quantprogramming.com.

I Discovered it Through Lektor

By using Lektor I need to learn Jinja, the template engine used. And I have to say it has the addons that I had always wished for in Django's template language (swig). For example there is else for the for loop, which applies when there is nothing to loop over.

And while looking for a tag system I came across lektor-tags which looks like the tag power and simplicity I need. While trying to get it to run, I came across this piece of Jinja template:

{% for t in this.tags -%}
  #{{ t }}
{% endfor %}

and I saw this typo in there -%}, so I removed the -, since I thought it was a typo. Later I saw it more often, even in an if and in macro and before endmacro too. So I figured it was not a typo. And it wasn't :).

So I started hunting the documentation. I found places where -%} was used but it was never described what the dash was for. Until I finally found it, of course by accident. In the Jinja docs in the chapter Whitespace Control it says the following:

You can also strip whitespace in templates by hand. If you add a minus sign (-) to the start or end of a block (e.g. a For tag), a comment, or a variable expression, the whitespaces before or after that block will be removed:

{% for item in seq -%}
  {{ item }}
{%- endfor %}

What a blessing. Something we always wanted in a template language. I remember back in the django days, the verbose {{ spaceless }} was quite noisy. I think this is a smart, simple and great addition. I am just not sure if the trade-off in time I spent to figure out what the magic is good for could be improved. Suggestions and ideas welcome.