jinja-typed-template 0.2.0

Static type checking for Jinja templates

jinja-typed-template

Type-safe Jinja2 templates with compile-time validation — keep your Python code and Jinja templates in sync, fail fast on mismatches, and never ship a broken template again.

Installation

pip install jinja-typed-template

For Flask support:

pip install jinja-typed-template[flask]

Quick Start

1. Annotate your template

Add a header comment declaring every variable the template uses, along with its type:

{#
    name: str
    age: int
    items: list[str]
#}
<h1>Hello {{ name }}!</h1>
<p>You are {{ age }} years old.</p>
<ul>
  {% for item in items %}
    <li>{{ item }}</li>
  {% endfor %}
</ul>

2. Define a typed template class

Subclass TypedTemplate, set the template name, and declare fields with type hints:

from jinja_typed_template import TypedTemplate, env_context
from jinja2 import Environment, FileSystemLoader

class ProfileTemplate(TypedTemplate):
    __template_name__ = "profile.html"
    name: str
    age: int
    items: list[str]

3. Render safely

Wrap usage in env_context to bind a Jinja2 environment, then instantiate and render:

env = Environment(loader=FileSystemLoader("templates"))

with env_context(env):
    t = ProfileTemplate(name="Alice", age=30, items=["apples", "bananas"])
    print(t.render())

What Gets Validated

Every TypedTemplate instance is validated at construction time (__post_init__). Three checks run automatically:

Check	Catches
Type checking	T(name=123) when name: str — raises TypeError
Variable matching	Missing or extra fields vs. what the template actually uses — raises ValueError
Header comment consistency	Header declares name: int but class says name: str — raises TypeError

This means you catch mismatches the moment you create the object, not when a user hits the page.

API Reference

TypedTemplate

Base class. Subclass it, set __template_name__, and declare typed fields.

Member	Description
render()	Render the template to a string
context_dict	Dict of all field names → values
copy_updating(**kwargs)	Return a new instance with some fields replaced (instances are frozen/immutable)

env_context(env)

Context manager that binds a Jinja2 Environment for the current thread/context. Required before creating or rendering any TypedTemplate.

from jinja_typed_template import env_context

with env_context(env):
    t = MyTemplate(...)

TypedTemplateExtension (Flask)

Flask extension that automatically pushes the app's Jinja2 environment into the context on every request. Context-processor variables are also discovered and excluded from validation, so they can appear in templates without being declared in the header.

from flask import Flask
from jinja_typed_template.extension_flask import TypedTemplateExtension

app = Flask(__name__)
ext = TypedTemplateExtension()
ext.init_app(app)

# Context-processor variables are automatically recognised:
@app.context_processor
def inject_globals():
    return {"app_name": "MyApp"}

Now any TypedTemplate instantiated inside a request will just work — no manual env_context needed.

Header Comment Format

Place a Jinja2 comment at the very top of the template:

{#
    variable_name: TypeName
    another_var: list[str]
    optional_var: str = "default"
#}

• variable_name must match a field on your TypedTemplate subclass.
• TypeName uses a short-form representation (str, int, list[str], dict[str, int], etc.) and must match the Python type hint.
• Default values after = are stripped — they're documentation only.

License

MIT