types-for-jinja

A mostly invisible type checking and LSP extension for Jinja, JinjaX, and similar templates, built on the pyright, ty, or mypy you already run. Declare a template's context once, in a comment, and types-for-jinja generate writes a line-aligned Python stub for it. Your checker then reports every bad variable and attribute the template touches, at the template's own line, in the same run that checks the rest of your code. In the editor, the same stubs feed your Python language server, so templates get inline diagnostics, completion, and hover without a second toolchain. There is no new template language, Jinja renders unchanged, and nothing runs at render time by default.

The only dependency is jinja2. types-for-jinja never invokes a type checker itself, so template checks run under your checker, your version, and your configuration, including checker plugins such as the mypy pydantic plugin. The "mostly" in invisible is raw CI output naming the stub instead of the template; types-for-jinja remap and the editor mirror close that gap, and "Limitations" lists the rest.

types-for-jinja is deliberately narrow (Jinja2 plus the dialects Jinja's own parser reads). If your needs differ, there are alternatives to consider:

• TypeJinja type-checks dbt's Jinja against dbt's own IR and ships with the dbt fusion engine, so use it for dbt projects
• templ (Go), askama (Rust), and Twirl (Scala) compile templates into typed host-language functions, the ergonomic model this project borrows
• JinjaX adds component syntax to Jinja (the {#def #} header comes from JinjaX) and composes with types-for-jinja rather than replacing it
• djlint lints and formats template style rather than types, so it runs alongside rather than instead

30-second example

Add a header to a template naming its context:

{#def
from myapp.models import User
user: User
#}
<h1>Hello {{ user.naem }}</h1>
{% for item in user.items %}
  <li>{{ item.titel }}</li>
{% endfor %}

Generate the stubs, then run whichever checker the project already uses:

$ types-for-jinja generate templates/    # or just `generate`, with template_dirs set
types-for-jinja: Wrote 5 file(s) for 1 template(s)
$ ty check
_jinja_stubs/templates/greeting_html.py:5:5: error[unresolved-attribute] Object of type `User` has no attribute `naem`
_jinja_stubs/templates/greeting_html.py:7:9: error[unresolved-attribute] Object of type `Item` has no attribute `titel`
Found 2 diagnostics

Generated line N is template line N, so the line numbers are the template's own. Pipe through remap when you want the template's path and column too:

$ ty check | types-for-jinja remap
templates/greeting.html:5:14: error[unresolved-attribute] Object of type `User` has no attribute `naem`
templates/greeting.html:7:10: error[unresolved-attribute] Object of type `Item` has no attribute `titel`
Found 2 diagnostics

A pipeline hands back the filter's exit code rather than the checker's, so in CI let remap run the checker instead. It exits with the checker's status, and with ty's default format the source excerpt becomes the template's own line:

$ types-for-jinja remap -- ty check
error[unresolved-attribute]: Object of type `User` has no attribute `naem`
 --> templates/greeting.html:5:14
  |
5 | <h1>Hello {{ user.naem }}</h1>
  |              ^^^^^^^^^
  |

remap reads text output from ty, mypy, and pyright, plus pyright --outputjson, mypy --output json, and ty's gitlab and github formats. A path outside the stub tree passes through untouched, so piping a whole-project run leaves the project's own diagnostics exactly as the checker wrote them.

The {#def #} block is a plain Jinja comment, so the template renders exactly as before. The loop variable is narrowed to its element type, so item.titel is caught the same way user.naem is.

A macro takes its own {#def #} block, which types its parameters for both its body and its callers, across files:

{% macro field(label, value) %}
  {#def
  label: str
  value: str
  #}
  <label>{{ label }}</label><span>{{ value }}</span>
{% endmacro %}

Without that block the parameters stay untyped and only arity is checked.

JinjaX writes the whole context on one line, with commas between, defaults allowed, and untyped names allowed. Both spellings parse, and they can be mixed:

{#def action, method: str = "post", count: int = 0 #}

A name with no annotation is Any, so it is declared without being constrained. A default is carried into the generated wrapper's signature, so callers do not have to repeat it.

JinjaX component tags are checked against the component's own header. Point template_dirs at the component directory and a use is validated for missing required attributes and for attribute types:

{#def title: str, count: int = 0 #}   <!-- components/Card.jinja -->

<Card count={{ user.name }} class="wide" />

$ types-for-jinja remap -- ty check
templates/page.html.jinja:6:16: error[missing-argument] No argument provided for required parameter `title`
templates/page.html.jinja:6:7: error[invalid-argument-type] Expected `int`, found `str`

An attribute the component does not declare is not an error, because JinjaX forwards it to the component as attrs, which is what class="wide" above relies on.

A tag an extension adds does not parse until the extension is loaded, and an unparseable template is skipped entirely, so its own errors go unreported too. Declaring the extension is what makes the rest of the template checkable:

[tool.types_for_jinja]
extensions = ["do", "i18n", "loopcontrols"]

i18n also declares the names it injects (_, gettext, ngettext, pgettext, npgettext), so a template using them needs nothing else. Jinja's own globals (namespace, cycler, joiner, lipsum) are declared automatically when a template names one, including the {% set ns.total = ... %} form.

Jinja's built-in filters carry their return type, so a filtered expression is still checked: {{ items | length }} is an int, and {% for x in items | sort %} still knows what x is. Only the return type is pinned, because a filter catalog that guesses at argument types reports errors on correct templates. A filter the catalog does not know (yours, or one from an extension) falls back to Any.

Installation

uv add types-for-jinja      # or: pip install types-for-jinja

That installs jinja2 and nothing else. Bring your own checker: pyright, ty, and mypy are each verified against the generated stubs on every CI run, and anything that reads standard Python annotations should work the same way.

How it works

types-for-jinja generate parses each template with Jinja's own parser and writes a small Python module that exercises every expression the template uses, preserving nesting so your checker's scoping and narrowing mirror Jinja's. {% for item in items %} becomes a real for loop, so the checker infers the element type. The stub tree mirrors the template tree under _jinja_stubs/ (only the filename is mangled, because a module name cannot carry a template extension), and a manifest maps each stub back to its template. Environment globals such as static_url are declared once under [tool.types_for_jinja] in pyproject.toml so they never show up as undefined.

Commit the stubs and a fresh clone type-checks its templates with no types-for-jinja run at all. Add types-for-jinja generate --check to pre-commit or CI to fail when a stub is missing or out of date; the shipped pre-commit hook does exactly that.

Cross-file constructs resolve at generation time: a child checks against its whole {% extends %} chain, an {% include %} body checks against the including template's context, and {% import %}-ed macros carry their own {#def #} types to their callers.

In your editor

The stubs are ordinary workspace Python, so the Python language server you already run flags them with no setup: open the stub and the error is on the same line number as the template. Two layers make that invisible:

• The types-for-jinja-lsp server (the lsp extra) attaches to template buffers and regenerates the stub as you type, debounced, so your Python checker re-checks it live before you save. It also completes and describes the typed context in the template itself: the names visible at the cursor, the members of their types after a ., built-in filters after | (with return types), tests after is, and tags after {%, plus hover for all of them.
• A thin mirror republishes the stub's diagnostics onto the template buffer, line for line, so errors appear inline in the template with your checker's own codes. Mirroring has to live in the editor, because one language server cannot read another server's diagnostics. editors/nvim ships it for Neovim. VS Code, Cursor, Zed, and Helix need the same layer against their own diagnostic APIs, and none of that is built.

Attribute completion asks a Python language server what the expression's type offers, taking the first of pyright, basedpyright, ty server, pylsp, or jedi found on PATH. Pin one with [tool.types_for_jinja] language_server if you run several. When none is installed, member completion is simply absent and everything else still works.

Configuration

Everything lives under [tool.types_for_jinja] in pyproject.toml, and every setting has a working default.

[tool.types_for_jinja]
imports = ["from collections.abc import Callable"]
template_dirs = ["myapp:templates"]

[tool.types_for_jinja.globals]
static_url = "Callable[[str], str]"
current_route = "str"

Suppressing a diagnostic

An inline {# type: ignore #} in the template becomes a blanket ignore comment on the generated line. Checkers spell that differently, so name yours if you run only one:

[tool.types_for_jinja]
suppression = "ty" # portable (default), mypy, pyright, or ty

The default emits # type: ignore, which pyright, ty, and mypy all honour. Naming a checker emits only what that checker reads, so pyright writes # pyright: ignore and mypy will not honour it. Suppression is blanket per line rather than per code, because rule codes differ between checkers and a wrong one fails to suppress.

Typed render calls (optional)

generate types the inside of a template. types-for-jinja wrapper types the call site, so render_profile(porfile=...) fails your checker the same way a typo in the template body does:

$ types-for-jinja wrapper templates/ -o myapp/_render \
    --env-import 'from myapp.templating import env as _env'
types-for-jinja: Wrote 3 of 3 wrapper file(s)

Each template gets one function whose signature is its {#def #} header, and whose body calls Jinja unchanged:

@beartype
def render_profile(*, profile: Profile) -> Markup:
    """Render profile.html.jinja with a checked context."""
    return Markup(_env.get_template('profile.html.jinja').render(profile=profile))

Set the options once in pyproject.toml instead of passing them every run:

[tool.types_for_jinja]
template_dirs = ["myapp/templates"]

[tool.types_for_jinja.wrapper]
env_import = "from myapp.templating import env as _env"
out_dir = "myapp/_render"
validator = "beartype"

template_dirs is what makes the generated get_template() argument match the name your loader uses. Run types-for-jinja wrapper --check in CI or a pre-commit hook to fail when a generated wrapper no longer matches its template.

Web apps usually return a response rather than Markup. Point --return-type and --return-import (or return_type and return_import in the config) at your framework's class and the wrapper calls it instead:

def render_profile(*, profile: Profile) -> HTMLResponse:
    """Render profile.html.jinja with a checked context."""
    return HTMLResponse(_env.get_template('profile.html.jinja').render(profile=profile))

A helper that does real work before rendering, or sets a status code, stays hand-written and calls the generated function for the render itself.

Runtime checking

--validator adds render-time enforcement on top: beartype checks the value against the annotation and raises, Pydantic parses and coerces it through a TypeAdapter. Both work whether your context types are dataclasses or Pydantic models. The default is none, because the static check costs nothing at runtime. Pydantic can hand the template a new coerced object, so the value you pass is not always the value rendered; beartype leaves the object alone. A runnable proof of both lives in examples/runtime.

Scope

types-for-jinja checks anything Jinja's own parser reads: plain Jinja2, JinjaX, and the templates in Flask, Litestar, FastAPI, Copier, and Cookiecutter projects. A superset that changes Jinja's delimiters declares them once, using the same names jinja2.Environment uses:

[tool.types_for_jinja.syntax]
variable_start_string = "[["
variable_end_string = "]]"

Copier and Cookiecutter put Jinja expressions in directory names. A stub tree mangles a directory name that is not already a valid identifier, so template/{{ module_name }}/__init__.py.jinja is checkable and still reported at its real path. Set template_globs when the templates are not .html, .jinja, or .j2:

[tool.types_for_jinja]
template_dirs = ["{{cookiecutter.project_slug}}"]
template_globs = ["*.md", "*.py", "*.toml"]

Templates shipped inside an installed package (what jinja2.PackageLoader loads) are reachable with a package:subdirectory entry in template_dirs, alongside plain paths:

[tool.types_for_jinja]
template_dirs = ["myapp:templates", "local/templates"]

Out of scope on purpose: Ansible, Salt, and dbt (untyped runtime contexts and large custom filter libraries, and dbt already has TypeJinja), engines not hosted in Python (Nunjucks, Twig, Liquid, Handlebars), and Python engines with different lookup semantics (Django's DTL, Mako, Chameleon). DESIGN gives the reasoning for each.

Limitations

• Raw checker output names the stub, not the template. The line number is the template's own, and the stub tree mirrors the template tree, so the mapping reads at a glance; remap recovers the path and column for CI logs, and the editor mirror recovers them inline. Python has no equivalent of Go's //line directive, which is why the path cannot be fixed at the source.
• Stubs must be regenerated when templates change. generate --check in pre-commit or CI catches a stale one; the LSP regenerates on edit.
• A template with no line-aligned form (rare; measured under 3% on real template sets) falls back to # L<n> markers, which remap and the mirror still read, and raw checker output does not.
• Filter and test argument types are unchecked; only built-in return types are pinned, and unknown filters widen to Any.
• A JinjaX component tag that cannot be resolved to a file is skipped, which includes any tag carrying a catalog prefix (<ui:Button />), because the prefix maps to a search path that lives in the catalog rather than in the template.
• Only jinja2's own extensions can be declared. A project-defined extension would mean importing project code to parse a template, and a custom tag's meaning is not inferable from its parser hook, so those templates are skipped with a warning.
• Templates that exist only behind a DictLoader or a database still need a copy on disk to be checked.

Project Status

Early and moving. DESIGN holds the settled decisions, the scope boundary, and the measurements they rest on; BLUE_SKY holds unscheduled work and why each item is waiting. See also the Open Issues and the CODE_TAG_SUMMARY. For release history, see the CHANGELOG.

Contributing

We welcome pull requests! For your pull request to be accepted smoothly, we suggest that you first open a GitHub issue to discuss your idea. For resources on getting started with the code base, see the below documentation:

• DEVELOPER_GUIDE
• STYLE_GUIDE

Code of Conduct

We follow the Contributor Covenant Code of Conduct.

Open Source Status

We try to reasonably meet most aspects of the "OpenSSF scorecard" from Open Source Insights

Responsible Disclosure

If you have any security issue to report, please contact the project maintainers privately. You can reach us at dev.act.kyle@gmail.com.

License

LICENSE