Template::Jinja2

Complete Jinja2 template engine for Raku, targeting compatibility with the Python reference implementation (Jinja2 3.1). Designed for HuggingFace chat templates and general-purpose templating.

Synopsis

use Template::Jinja2;

my $env = Template::Jinja2.new;
my $tmpl = $env.from-string('Hello {{ name }}!');
say $tmpl.render(name => 'World');  # Hello World!

HuggingFace Chat Templates

use Template::Jinja2;
use JSON::Fast;

my $config = from-json('tokenizer_config.json'.IO.slurp);
my $chat-template = $config<chat_template>;

my $env = Template::Jinja2.new;
my $result = $env.from-string($chat-template).render(
    messages => [
        { role => 'system', content => 'You are helpful.' },
        { role => 'user', content => 'Hello!' },
    ],
    bos_token => '<s>',
    add_generation_prompt => True,
);

Produces byte-identical output to Python Jinja2 for ChatML, Llama 3, Mistral, Gemma 2, Zephyr, and Cohere Command A templates.

Features

Tags

All 15 Jinja2 tag types plus loop controls:

• {% if %} / {% elif %} / {% else %} / {% endif %}

• {% for %} / {% else %} / {% endfor %} with full loop context

• {% set %} / {% endset %} with filter chains and namespace support

• {% with %} / {% endwith %}

• {% block %} / {% endblock %} with scoped blocks

• {% extends %} / {% include %}

• {% import %} / {% from ... import %}

• {% macro %} / {% endmacro %} with defaults, varargs, kwargs

• {% call %} / {% endcall %} with parameterized caller

• {% filter %} / {% endfilter %}

• {% do %} / {% raw %} / {# comment #}

• {% autoescape %} / {% endautoescape %}

• {% break %} / {% continue %}

Expressions

• Arithmetic: +, -, *, /, //, %, **

• Comparison: ==, !=, <, >, <=, >=>, chained

• Logical: and, or, not

• Membership: in, not in

• Identity: is, is not

• Ternary: expr if test else expr

• String concat: ~

• Filters: value | filter(args)

• Subscript: items[0], items[-1], items[1:3], items[::-1]

• Dot access: obj.attr, list.0

• Literals: strings, integers, floats, hex/octal/binary, scientific notation, lists, dicts, tuples

Python Methods

Jinja2 has no method syntax of its own: value.method() resolves the name on the underlying Python object and calls it. Templates in the wild — HuggingFace chat templates especially — rely on that constantly, so the Python str and dict methods are implemented with Python semantics, not those of Raku's similarly-named methods.

{%- for m in messages -%}
    {#- strip() as both a guard and as output -#}
    {%- if m.content.strip() -%}
        <|{{ m.role }}|>{{ m.content.strip() }}
    {%- endif -%}

    {#- reasoning split, straight out of the GLM chat template -#}
    {%- if '</think>' in m.content -%}
        {%- set reasoning = m.content.split('</think>')[0].split('<think>')[-1] -%}
    {%- endif -%}

    {#- get() with a default, straight out of the Kimi chat template -#}
    {%- set name = m.get('name') or m['role'] -%}
    {%- set rc = m.get('reasoning', m.get('reasoning_content', '')) -%}
{%- endfor -%}

{#- dict iteration for tool schemas -#}
{%- for k, v in tool.items() -%}"{{ k }}": {{ v | tojson }}{%- endfor -%}

String methods:

• strip(chars?), lstrip(chars?), rstrip(chars?) — no argument strips whitespace; with an argument it strips any of the given characters, so "abcHIcba".strip("abc") is "HI", not a prefix trim

• startswith(prefix), endswith(suffix) — the argument may be a tuple (or list) of candidates, any of which may match: name.startswith(('sys_', 'usr_'))

• replace(old, new, count?) — replaces every occurrence unless count is given; an empty old inserts between every character, as in Python

• split(sep?, maxsplit?) — with no separator, splits on runs of whitespace and drops empty fields ("a b c".split() is ['a', 'b', 'c'], however many blanks separate the words); with a separator it is a literal split that keeps them ("a||b".split("|") is ['a', '', 'b']). maxsplit caps the number of splits and leaves the remainder verbatim

• lower(), upper() — Unicode-aware case folding

• title() — Python's str.title(): the first letter after every non-letter is uppercased, so "it's a test" becomes "It'S A Test". Note this is deliberately not the title filter, which follows Jinja2's gentler word splitting and yields "It's A Test"

• find(needle, start?, end?) — the first index, or -1 when absent (never an undefined value); negative start/end count from the end

Dict methods:

• items() — (key, value) tuples, ready for {% for k, v in d.items() %}

• keys(), values()

• get(key), get(key, default) — a missing key yields None (falsy, renders as None) with no default, so m.get('name') or m['role'] works exactly as it does in Python

Method results chain, feed filters and tests, and bind through {% set %}:

{{ msg.content.strip().split('\n')[0] }}
{{ msg.content.strip() | tojson }}
{% set parts = ref.split('|') %}{{ parts[0] }}/{{ parts[-1] }}

Two deliberate divergences from CPython, both consequences of a Raku Hash having no insertion order to preserve:

• items(), keys() and values() iterate in sorted-key order, which is what {% for k in d %}, |items and |dictsort already do in this engine. Output is deterministic, but a template that rebuilds JSON by iterating a dict emits its keys sorted rather than in the order they were written.

• {{ d.keys() }} renders as a plain list (['a', 'b']) rather than Python's dict_keys(['a', 'b']) view repr. Iterating, indexing and filtering are unaffected.

Anything else raises a TemplateRuntimeError naming the method and the receiver's type — 'str' object has no method 'casefold' — rather than reaching a same-named Raku method or leaking an internal error. Calling a method on an undefined value raises UndefinedError identifying the expression that produced it.

Filters (52)

abs, attr, batch, capitalize, center, count, d/default, dictsort, e/escape, filesizeformat, first, float, forceescape, format, groupby, indent, int, items, join, last, length, list, lower, lstrip, map, max, min, pprint, random, reject, rejectattr, replace, reverse, round, rstrip, safe, select, selectattr, slice, sort, string, striptags, sum, title, tojson, trim, truncate, unique, upper, urlencode, urlize, wordcount, wordwrap, xmlattr

Tests (20+)

defined, undefined, none, boolean, integer, float, number, string, sequence, mapping, iterable, callable, sameas, eq/equalto, ne, lt/lessthan, le, gt/greaterthan, ge, even, odd, divisibleby, in, true, false, lower, upper

Template Inheritance

{# base.html #}
<html>{% block content %}default{% endblock %}</html>

{# child.html #}
{% extends "base.html" %}
{% block content %}{{ super() }} + override{% endblock %}

Supports multi-level inheritance with super() chaining.

Whitespace Control

{%- trim left -%}    {# trim both sides #}
{%+ no_lstrip +%}    {# disable lstrip/trim_blocks #}

my $env = Template::Jinja2.new(:trim-blocks, :lstrip-blocks);

Custom Delimiters

# ERB-style
my $env = Template::Jinja2.new(
    :block-start('<%'), :block-end('%>'),
    :variable-start('<%='), :variable-end('%>'),
    :comment-start('<%#'), :comment-end('%>'));

# PHP-style
my $env = Template::Jinja2.new(
    :block-start('<?'), :block-end('?>'),
    :variable-start('<?='), :variable-end('?>'),
    :comment-start('<!--'), :comment-end('-->'));

Loaders

use Template::Jinja2::Loader;

# File system
my $env = Template::Jinja2.new(
    loader => FileSystemLoader.new(searchpath => 'templates/'));

# In-memory
my $env = Template::Jinja2.new(
    loader => DictLoader.new(templates => {
        'base.html' => '...',
        'child.html' => '...',
    }));

Configuration

my $env = Template::Jinja2.new(
    :autoescape,                  # Auto-escape {{ }} output
    :trim-blocks,                 # Strip newline after block tags
    :lstrip-blocks,               # Strip leading whitespace before block tags
    :!keep-trailing-newline,      # Strip trailing newline from output
    :line-statement-prefix('#'),  # Line-based block syntax
    :line-comment-prefix('##'),   # Line-based comments
    globals => { site => 'My Site' },
    filters => { custom => sub ($v) { ... } },
    tests => { custom => sub ($v) { ... } },
);

Architecture

Template String -> [Lexer] -> Tokens -> [Parser] -> AST -> [Renderer] + Context -> Output

• Lexer — Raku grammar tokenizer (default delimiters) or regex chunker (custom delimiters)

• Parser — Pratt-style expression parser with block tag dispatch

• AST — 30+ node types covering all Jinja2 constructs

• Renderer — AST walker with expression evaluation, scope chain, filter/test/method dispatch

• Methods — Python str/dict method semantics for value.method() calls

• Context — Scope stack with Undefined sentinel and Namespace support

Testing

644 tests across 24 test files, including:

• Ported tests from the Python Jinja2 reference test suite

• Real-world HuggingFace chat template validation

• Byte-identical output verification against Python Jinja2 3.1.6

• Python str/dict method semantics, expectation-for-expectation against CPython

# Run all tests
prove6 -I lib t/

# Run a single test
raku -I lib t/09-huggingface.rakutest

Author

Matt Doughty

License

Artistic-2.0