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172 lines
5.8 KiB
Plaintext
172 lines
5.8 KiB
Plaintext
==========================================
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How to implement a custom template backend
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==========================================
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Custom backends
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---------------
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Here's how to implement a custom template backend in order to use another
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template system. A template backend is a class that inherits
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``django.template.backends.base.BaseEngine``. It must implement
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``get_template()`` and optionally ``from_string()``. Here's an example for a
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fictional ``foobar`` template library::
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from django.template import TemplateDoesNotExist, TemplateSyntaxError
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from django.template.backends.base import BaseEngine
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from django.template.backends.utils import csrf_input_lazy, csrf_token_lazy
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import foobar
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class FooBar(BaseEngine):
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# Name of the subdirectory containing the templates for this engine
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# inside an installed application.
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app_dirname = "foobar"
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def __init__(self, params):
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params = params.copy()
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options = params.pop("OPTIONS").copy()
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super().__init__(params)
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self.engine = foobar.Engine(**options)
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def from_string(self, template_code):
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try:
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return Template(self.engine.from_string(template_code))
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except foobar.TemplateCompilationFailed as exc:
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raise TemplateSyntaxError(exc.args)
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def get_template(self, template_name):
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try:
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return Template(self.engine.get_template(template_name))
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except foobar.TemplateNotFound as exc:
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raise TemplateDoesNotExist(exc.args, backend=self)
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except foobar.TemplateCompilationFailed as exc:
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raise TemplateSyntaxError(exc.args)
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class Template:
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def __init__(self, template):
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self.template = template
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def render(self, context=None, request=None):
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if context is None:
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context = {}
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if request is not None:
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context["request"] = request
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context["csrf_input"] = csrf_input_lazy(request)
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context["csrf_token"] = csrf_token_lazy(request)
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return self.template.render(context)
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See `DEP 182`_ for more information.
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.. _template-debug-integration:
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Debug integration for custom engines
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------------------------------------
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The Django debug page has hooks to provide detailed information when a template
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error arises. Custom template engines can use these hooks to enhance the
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traceback information that appears to users. The following hooks are available:
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.. _template-postmortem:
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Template postmortem
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~~~~~~~~~~~~~~~~~~~
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The postmortem appears when :exc:`~django.template.TemplateDoesNotExist` is
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raised. It lists the template engines and loaders that were used when trying to
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find a given template. For example, if two Django engines are configured, the
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postmortem will appear like:
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.. image:: _images/postmortem.png
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Custom engines can populate the postmortem by passing the ``backend`` and
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``tried`` arguments when raising :exc:`~django.template.TemplateDoesNotExist`.
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Backends that use the postmortem :ref:`should specify an origin
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<template-origin-api>` on the template object.
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Contextual line information
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If an error happens during template parsing or rendering, Django can display
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the line the error happened on. For example:
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.. image:: _images/template-lines.png
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Custom engines can populate this information by setting a ``template_debug``
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attribute on exceptions raised during parsing and rendering. This attribute is
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a :class:`dict` with the following values:
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* ``'name'``: The name of the template in which the exception occurred.
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* ``'message'``: The exception message.
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* ``'source_lines'``: The lines before, after, and including the line the
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exception occurred on. This is for context, so it shouldn't contain more than
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20 lines or so.
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* ``'line'``: The line number on which the exception occurred.
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* ``'before'``: The content on the error line before the token that raised the
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error.
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* ``'during'``: The token that raised the error.
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* ``'after'``: The content on the error line after the token that raised the
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error.
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* ``'total'``: The number of lines in ``source_lines``.
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* ``'top'``: The line number where ``source_lines`` starts.
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* ``'bottom'``: The line number where ``source_lines`` ends.
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Given the above template error, ``template_debug`` would look like::
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{
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"name": "/path/to/template.html",
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"message": "Invalid block tag: 'syntax'",
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"source_lines": [
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(1, "some\n"),
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(2, "lines\n"),
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(3, "before\n"),
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(4, "Hello {% syntax error %} {{ world }}\n"),
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(5, "some\n"),
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(6, "lines\n"),
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(7, "after\n"),
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(8, ""),
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],
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"line": 4,
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"before": "Hello ",
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"during": "{% syntax error %}",
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"after": " {{ world }}\n",
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"total": 9,
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"bottom": 9,
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"top": 1,
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}
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.. _template-origin-api:
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Origin API and 3rd-party integration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Django templates have an :class:`~django.template.base.Origin` object available
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through the ``template.origin`` attribute. This enables debug information to be
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displayed in the :ref:`template postmortem <template-postmortem>`, as well as
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in 3rd-party libraries, like the :pypi:`Django Debug Toolbar
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<django-debug-toolbar>`.
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Custom engines can provide their own ``template.origin`` information by
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creating an object that specifies the following attributes:
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* ``'name'``: The full path to the template.
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* ``'template_name'``: The relative path to the template as passed into the
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template loading methods.
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* ``'loader_name'``: An optional string identifying the function or class used
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to load the template, e.g. ``django.template.loaders.filesystem.Loader``.
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.. _DEP 182: https://github.com/django/deps/blob/main/final/0182-multiple-template-engines.rst
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