#9413 Declare py namespace on document root to generate valid xml

[nemani]

Subject: Declare py namespace in XML writer.

Feature or Bugfix

• Bugfix

Purpose

• Autodoc uses py:class, py:module, etc as attributes for nodes. This causes errors as the generated xml does not have a namespace for py declared. This PR fixes this by declaring a xmlns:py at the document level.

Relates

• Autodoc generates invalid xml  #9413

Please recommend what URI to use for the declaration of the py xml namespace

[tk0miya]

LGTM with nits.

[tk0miya]

I think it's not good to set the URL of #9413. I'd like to use a better one for this. What kind of URL should we use instead? document? project page (GitHub)?

[nemani]

Maybe we can point to the documentation of the domains?

[tk0miya]

We can point only the URL of the "bundled" domains: https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html
But this document does not contains the 3rd party domains. For example, sphinx-http-domain: https://pypi.org/project/sphinx-http-domain/.

[nemani]

Maybe we can keep it as an attribute in the domain class with default pointing to the "bundled" domains doc?

[tk0miya]

The "py" namespace comes from the domains mark-up.
https://www.sphinx-doc.org/en/master/usage/restructuredtext/domains.html

And we have many kinds of domains (ex. C, C++, etc.). And developers can add them via API. So it would be better to add it with such code:

Suggested change

	doctree.document.attributes["xmlns:py"] = "https://github.com/sphinx-doc/sphinx/issues/9413"
	for domain in self.env.domains.values():
	xmlns = "xmlns:" + domain.name
	doctree.document.attributes[xmlns] = "https://URL/to/somewhere"

[tk0miya]

Now I merged #10100 instead. Thank you for your contribution!