==========================================
How to implement a custom template backend
==========================================

Custom backends
---------------

Here's how to implement a custom template backend in order to use another
template system. A template backend is a class that inherits
``django.template.backends.base.BaseEngine``. It must implement
``get_template()`` and optionally ``from_string()``. Here's an example for a
fictional ``foobar`` template library::

    from django.template import TemplateDoesNotExist, TemplateSyntaxError
    from django.template.backends.base import BaseEngine
    from django.template.backends.utils import csrf_input_lazy, csrf_token_lazy

    import foobar


    class FooBar(BaseEngine):
        # Name of the subdirectory containing the templates for this engine
        # inside an installed application.
        app_dirname = "foobar"

        def __init__(self, params):
            params = params.copy()
            options = params.pop("OPTIONS").copy()
            super().__init__(params)

            self.engine = foobar.Engine(**options)

        def from_string(self, template_code):
            try:
                return Template(self.engine.from_string(template_code))
            except foobar.TemplateCompilationFailed as exc:
                raise TemplateSyntaxError(exc.args)

        def get_template(self, template_name):
            try:
                return Template(self.engine.get_template(template_name))
            except foobar.TemplateNotFound as exc:
                raise TemplateDoesNotExist(exc.args, backend=self)
            except foobar.TemplateCompilationFailed as exc:
                raise TemplateSyntaxError(exc.args)


    class Template:
        def __init__(self, template):
            self.template = template

        def render(self, context=None, request=None):
            if context is None:
                context = {}
            if request is not None:
                context["request"] = request
                context["csrf_input"] = csrf_input_lazy(request)
                context["csrf_token"] = csrf_token_lazy(request)
            return self.template.render(context)

See `DEP 182`_ for more information.

.. _template-debug-integration:

Debug integration for custom engines
------------------------------------

The Django debug page has hooks to provide detailed information when a template
error arises. Custom template engines can use these hooks to enhance the
traceback information that appears to users. The following hooks are available:

.. _template-postmortem:

Template postmortem
~~~~~~~~~~~~~~~~~~~

The postmortem appears when :exc:`~django.template.TemplateDoesNotExist` is
raised. It lists the template engines and loaders that were used when trying to
find a given template. For example, if two Django engines are configured, the
postmortem will appear like:

.. image:: _images/postmortem.png

Custom engines can populate the postmortem by passing the ``backend`` and
``tried`` arguments when raising :exc:`~django.template.TemplateDoesNotExist`.
Backends that use the postmortem :ref:`should specify an origin
<template-origin-api>` on the template object.

Contextual line information
~~~~~~~~~~~~~~~~~~~~~~~~~~~

If an error happens during template parsing or rendering, Django can display
the line the error happened on. For example:

.. image:: _images/template-lines.png

Custom engines can populate this information by setting a ``template_debug``
attribute on exceptions raised during parsing and rendering. This attribute is
a :class:`dict` with the following values:

* ``'name'``: The name of the template in which the exception occurred.

* ``'message'``: The exception message.

* ``'source_lines'``: The lines before, after, and including the line the
  exception occurred on. This is for context, so it shouldn't contain more than
  20 lines or so.

* ``'line'``: The line number on which the exception occurred.

* ``'before'``: The content on the error line before the token that raised the
  error.

* ``'during'``: The token that raised the error.

* ``'after'``: The content on the error line after the token that raised the
  error.

* ``'total'``: The number of lines in ``source_lines``.

* ``'top'``: The line number where ``source_lines`` starts.

* ``'bottom'``: The line number where ``source_lines`` ends.

Given the above template error, ``template_debug`` would look like::

    {
        "name": "/path/to/template.html",
        "message": "Invalid block tag: 'syntax'",
        "source_lines": [
            (1, "some\n"),
            (2, "lines\n"),
            (3, "before\n"),
            (4, "Hello {% syntax error %} {{ world }}\n"),
            (5, "some\n"),
            (6, "lines\n"),
            (7, "after\n"),
            (8, ""),
        ],
        "line": 4,
        "before": "Hello ",
        "during": "{% syntax error %}",
        "after": " {{ world }}\n",
        "total": 9,
        "bottom": 9,
        "top": 1,
    }

.. _template-origin-api:

Origin API and 3rd-party integration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Django templates have an :class:`~django.template.base.Origin` object available
through the ``template.origin`` attribute. This enables debug information to be
displayed in the :ref:`template postmortem <template-postmortem>`, as well as
in 3rd-party libraries, like the `Django Debug Toolbar`_.

Custom engines can provide their own ``template.origin`` information by
creating an object that specifies the following attributes:

* ``'name'``: The full path to the template.

* ``'template_name'``: The relative path to the template as passed into the
  template loading methods.

* ``'loader_name'``: An optional string identifying the function or class used
  to load the template, e.g. ``django.template.loaders.filesystem.Loader``.

.. _DEP 182: https://github.com/django/deps/blob/main/final/0182-multiple-template-engines.rst
.. _Django Debug Toolbar: https://github.com/jazzband/django-debug-toolbar/
