Element access to a slice of a list returns undefined

[Algy]

Description

Accessing an element of a slice of an array seems impossible.

Reproduction steps

Let us see this example:

{% set messages = ["first", "second", "third"] %}
FIRST: {{ messages[0] }}
{% set messages = messages[1:] %}
SECOND: {{ messages[0] }}

Result:

FIRST: first
SECOND: 

A simpler case goes like:

{{ [1,2,3][1:][0] }}

which should print "2". However, it returns undefined thus a blank string.

You can easily reproduce them in the minijinja playgorund.

What did you expect

It should work as if I accessed an element of a sub-list. It seems it works in the original Jinja2 written in Python.

[mitsuhiko]

This is happening because slicing returns an iterator and not a sequence. You can solve this by using |list:

{{ ([1,2,3][1:]|list)[0] }}

This is not how it works in Jinja2 but I wonder if it's reasonable to stringify this as it seems a bit of a weird edge case. However this also makes me wonder if it should not be possible to index into an iterable. Since they are (usually) restartable in MiniJinja that wouldn't be the worst in the world.

[Algy]

I can see why you make the slice operation return an iterator (or iterable?) instead of what is expected in python jinja2, at least for the sake of performance in Rust.

You might wonder why this issue (even practically) matters. Let me explain the reason how I came across this edge case issue. This is exactly how the "chat template" of Llama 3.1 works, which is one of most famous open-source LLMs. A chat template is an initial template to feed from which a LLM auto-generate more texts. To optimize the LLM inference system, I'm using Minijinja to render it instead of using plain python jinja2 for better system throughput.

However, the actual use of jinja template is quite biased into the python library.

For example of the chat template of Llama 3.1 above (you can see this in the value of the field chat_template in tokenizer_config.json in its huggingface repository):

...
{%- if messages[0]['role'] == 'system' %}
    {%- set system_message = messages[0]['content']|trim %}
    {%- set messages = messages[1:] %}
{%- else %}
    {%- set system_message = "" %}
{%- endif %}

...

{#- Custom tools are passed in a user message with some extra guidance #}
{%- if tools_in_user_message and not tools is none %}
    {#- Extract the first user message so we can plug it in here #}
    {%- if messages | length != 0 %}
        {%- set first_user_message = messages[0]['content']|trim %}
        {%- set messages = messages[1:] %}
    {%- else %}
        {{- raise_exception("Cannot put tools in the first user message when there's no first user message!") }}
{%- endif %} 
{%- endif %}
...

And you'll see the point why the access of a sliced list actually mattered in a jinja2 template.

[mitsuhiko]

Will work in the next release.