app.registry.get_translator_class returns 'property' objects for certain builders.

[domdfcoding]

Describe the bug

sphinx.builders.html.StandaloneHTMLBuilder uses a property for default_translator_class, but in the base class it's a conventional variable. This means that code using app.registry.get_translator_class for any builder that subclasses StandaloneHTMLBuilder is given a property object rather than the actual nodes.NodeVisitor class.

How to Reproduce

The example conf.py below illustrates what I'm doing:

from docutils.nodes import paragraph

def setup(app):
    builder = app.registry.builders["epub"]
    epub_translator = app.registry.get_translator_class(builder)
    app.add_node(paragraph, epub=(epub_translator.visit_paragraph, epub_translator.depart_paragraph))

Running sphinx-build -M html . ./build -T:

Running Sphinx v4.1.1

Traceback (most recent call last):
  File "venv/lib/python3.8/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "venv/lib/python3.8/site-packages/sphinx/application.py", line 251, in __init__
    self.config.setup(self)
  File "conf.py", line 7, in setup
    app.add_node(paragraph, epub=(epub_translator.visit_paragraph, epub_translator.depart_paragraph))
AttributeError: 'property' object has no attribute 'visit_paragraph'

Exception occurred:
  File "conf.py", line 7, in setup
    app.add_node(paragraph, epub=(epub_translator.visit_paragraph, epub_translator.depart_paragraph))
AttributeError: 'property' object has no attribute 'visit_paragraph'
The full traceback has been saved in /tmp/sphinx-err-46tp_ln9.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

Expected behavior

The default_translator_class attribute, and app.registry.get_translator_class, should always returns a nodes.NodeVisitor class, per the type annotations on both.

Your project

N/A

Screenshots

No response

OS

Ubuntu 20.04

Python version

3.8.10

Sphinx version

2.0.0 - 4.1.1

Sphinx extensions

None

Extra tools

None

Additional context

The offending code is here:

sphinx/sphinx/builders/html/__init__.py

Lines 335 to 340 in 07598f0

	@property
	def default_translator_class(self) -> Type[nodes.NodeVisitor]: # type: ignore
	if not html5_ready or self.config.html4_writer:
	return HTMLTranslator
	else:
	return HTML5Translator

I suspect the # type: ignore is there because mypy was warning about exactly this issue. If I remove the comment I get an (albeit cryptic) error:

sphinx/builders/html/__init__.py: note: In class "StandaloneHTMLBuilder":
sphinx/builders/html/__init__.py:336:5: error: Signature of "default_translator_class" incompatible with supertype "Builder"

mypy has determined the signature to be def (self: sphinx.builders.html.StandaloneHTMLBuilder) -> Type[docutils.nodes.NodeVisitor].

[tk0miya]

    builder = app.registry.builders["epub"]
    epub_translator = app.registry.get_translator_class(builder)

This is incorrect code. app.registry.get_translator_class() takes an instance of builder. But you passed a builder class, not an instance. I think the code that you want is this:

def setup(app):
    builder = app.registry.create_builder(app, "builder")
    epub_translator = app.registry.get_translator_class(builder)
    app.add_node(paragraph, epub=(epub_translator.visit_paragraph, epub_translator.depart_paragraph))

BTW, why do you want to get visitor functions? I think it's enough to call visit_paragraph() handler from your custom handler:

def epub_visit_my_node(self, node):
    self.visit_paragraph(node)

def setup(app):
    app.add_node(my_node, epub=(epub_visit_my_node, epub_depart_my_node))