February 2026

Fall Back to a Default Include File in MiniJinja

I'm working on upgrading the documentation for bitty. Everything is built with MiniJinja (rust's version of the Jinja template engine).

Very section has live test code with it. Most of the tests trigger the same way. A few don't. I wanted a way that I could use a script to generate the default approach and then override it when I need. I'm doing with in three steps:

Use a script to generate default files with names the begin with an underscore.
Manually make files with the same name minus the underscore when I need to customize something.
Have MiniJinja use the non-underscore version of the file if it exists. Otherwise, fall back to using the default one that starts with the character.

The file structure looks like this:

alfa-test/
  code_to_test.js
  _trigger.js
echo-test/
  code_to_test.js
  _trigger.js
  trigger.js

The goal is to use the trigger files:

alfa-test/_trigger.js
and
echo-test/trigger.js

I'm using this MiniJinja macro to make it happen:

[! macro get_content(folder, key) !]
    [! set default_content !]
        [! include [folder, "/_", key]|join("") !]
    [! endset !]
    [! set custom_content !]
        [! include [folder, "/", key]|join("") ignore missing !]
    [! endset !]
    [! if custom_content == "" !]
        [@ default_content @]
    [! else !]
        [@ custom_content @]
    [! endif !]
[! endmacro !]

It gets called like:

[@ get_content("alfa-test", "trigger.js") @]

[@ get_content("echo-test", "trigger.js") @]

In each case the base filename without the underscore is called. That's what gets used if it exists. Otherwise, the underscore fallback gets output.

This way I can just let the defaults do their thing then customize when necessary. Way better than having to duplicate everything myself.

-a

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Endnotes

In some cases it would be possible to use a single copy of the default file stored in a central location. That won't work for what I'm doing. Each of the default trigger files has a unique ID in it that's generated outside of MiniJinja.

The default Jinja/MiniJinja tokens are hard for me to scan quickly. I switch them out for the square bracket versions you see here (e.g. {{ }} becomes [@ @].