Writing a Jinja-inspired template library in Python

I've seen a large scale Mako codebase and the amount of business logic that had sneaked into the templates was horrifying - really made me double-down on my preference for limited template languages that help people avoid that temptation.

I helped design Django's template language and I now think we took keeping it restrictive a bit too far though - these days I prefer working with Jinja because it has better support for complex expressions and lets you call functions within your templates. That does mean it requires a bit more discipline to use responsibly though, but being able to control which functions are exposed to the template context helps a lot there.

Having never used Mako the appeal to me over Jinja is that it allows you to use real control structures. Jinja is a bit of a pain to debug and develop because it does some things slightly differently than you'd do in native Python so even though I can setup breakpoints and evaluate the application state I can't easily convert the code changes I make in the debugger back to the template. I get that there are reasons why Jinja and other templating systems guard against this, but it does have a cost.

I hate jinja. I really don’t get it. So here’s a complaining reply, so you hopefully won’t feel as alone.

When using jinja2 for website builders, it just feels like a poor version of old school php.

I’ve also seen it used for generating json/yaml config files, which to me is absolutely bunkers compared to just building a dictionary in Python and exporting it.

In D, vibe.d does this. It uses a pug like syntax for templates.

Edit: They've extracted the library into its own thing called diet-ng

https://vibed.org/

https://github.com/rejectedsoftware/diet-ng/

Most template languages for Lua generate functions, that will be called with their own environment. Having first-class environments makes that the "obvious" way of doing things.

For example, here's a 160LOC one I wrote a while back. [0]

[0] https://git.sr.ht/~shakna/lserver/tree/master/item/template....

Twig, inspired by Jinja2.

https://twig.symfony.com/

Yes, there is pongo2 [0] and my runner (basically a small Go software that runs the template engine) pongo2-runner [1].

I'm not the author of the library (pongo2), but I'm using pongo2-runner to dynamically create config files out of environment variables, with custom logic. Super recommended.

[0]: https://github.com/flosch/pongo2

[1]: https://github.com/swisscom/pongo2-runner