One of the big failing of Jinja in my opinion is that it's Python with
only instance methods, but Python has a weird, arbitrary-seeming historic division between basic functionality being implemented as global functions versus instance methods.
So if you want to replace a character in a string, that's fine, because `replace` is an instance method:
Changed plus to dot: {{ string.replace('+', '.') }}.
but if you want to take the
length of that string, you need a Jinga-specific filter to have been implemented, because getting the length is via a global function `len()` in Python. This fails:
My string is {{ len(string) }} long.
If you're just using Jinga to build some HTML or whatever and you were always in your own Python code right before the template it's not really a problem, 'cause you can just do it in said Python code. I happily used Jinga for several web apps for years.
I really started to hate it in Ansible though, where I'm often just trying to get a one-off little operation done that I would rather have obvious right there in the template source rather than having to bounce over to a one-off filter function (or invoke some other outside code), and I can do it in Jinja, but it just looks and feels like hell... which kinda defeats the purpose.
I feel like Jinja was an especially bad fit for Ansible, but as a templating language I feel like it's main strength is it's better than D'jango templates... but that doesn't matter much outside the D'Jango world.
As a logic-ful template language I think it's deficient because it omits the functional parts of Python, which - as discussed above regarding `len()` - are very core to Python, and Jinja adds them back in a different way that requires extra mental overhead to keep track of. I really like the pipes (filters) but it would have been nice to just keep the Python globals as well for consistency.
And if you really want to go logic-less with your templates, I think Mustache(-variants) would be the way to go since they're much more truly "logic-less" and they're supported across a plethora of different runtimes besides Python, giving your much more flexibility. I've never really had the patience / discipline / right use case for logic-less though, so I'm far from an expert in this area.
Jinja is just a really weird mix... it's not logic-less, but it's not Python... it's a funky half-logic mishmash. It obviously makes sense and feels right to the people that make it, and if it makes sense and feels right for you that's awesome, but I hope you can understand how could feel awkward to others.