Controlling whitespace in Jinja2 templates
For background perspective, Jinja is used as a templating system by various Python projects like Django, Flask, or Ansible.
From the Jinja2 documentation, there are two options to control whitespace in Jinja templates:
trim_blockslstrip_blocks
But actually, there's more to it. The "Template Designer Documentation" specifically for whitespace control is inadequate. That's why I wrote this article.
Jinja in Ansible[edit | edit source]
In Ansible's Template module, you can see in the source code
But what does this mean? What do these options do? And how do I use them in my Ansible templates?
trim blocks means the first newline after a block is removed (block, not variable tag!)
lstrip blocks [1] means leading spaces and tabs are stripped from the start of a line to a block.
So, I can read words, but without context nor further example and explanation, I can't decode what "from the start of a line to a block" means.
In the task[edit | edit source]
In at least v2.9 of Ansible, the Template module has options to specify both.
In the template file[edit | edit source]
As the very first line of your template file, you can use a magic comment directive to control the behavior of the Ansible Template module
#jinja2: trim_blocks: "true", lstrip_blocks: "true"[2]
By hand[edit | edit source]
I was so frustrated by the lack of clear examples that showed all variants, I created a project on GitHub called test-jinja that does exactly that. (Inspiration from https://github.com/ansible/ansible/pull/37478)
You could also use the jinja2 live parser project to interactively preview jinja.
If you're confused about Variable notation and how those are defined in templates, see https://jinja.palletsprojects.com/en/3.1.x/templates/#variables
Debugging[edit | edit source]
Just put {% debug %} somewhere in your template file. This didn't work for me in Ansible 2.9
More[edit | edit source]
Bloomreach Engagement is a commercial Web product that uses Jinja for templating. So they have a pretty good reference guide.== References ==
- ↑
lstripstands for "left strip". It is a function in Python, as well as R and Ruby. Likeltrimin PHP ortrimStartin JavaScript. nb. Jinja also has a built-in filter namedtrimthat is used to strip characters from the beginning and end of a string. - ↑ It's unclear whether the boolean value(s) to be quoted. I believe it was a bug that is now fixed - meaning they can be bare.
