After doing a make clean in Doc, the built documents no longer rendered correctly.  I tracked this down to the Sphinx warning I'd been ignoring for a while:

WARNING: 'default' html theme has been renamed to 'classic'. Please change your html_theme setting either to the new 'alabaster' default theme, or to 'classic' to keep using the old default.

Now, this seems problematic, because we don't know which version of Sphinx is going to be used to build the docs, so it doesn't seen correct to just change "inherit = default" to "inherit = classic" in tools/pydoctheme.conf.

Any sphinx experts know what the correct solution is here?  Do we change it and thus (IIUC) require 1.3.1 as the minimum Sphinx version?

Is the behavior different in default or 3.5 release and 2.7 release? 

The default branch has html_theme in Doc/conf.py https://hg.python.org/cpython/file/default/Doc/conf.py set to 'pydoctheme'.

The 2.7 branch has html_theme in Doc/conf.py https://hg.python.org/cpython/file/2.7/Doc/conf.py to 'default'.

Iss24400 made a recent change https://hg.python.org/cpython/rev/68996acdec6f.

I only checked this on 3.5 and 3.6, but 3.4 is using pydoctheme, and python2.7 is using default.  So presumably none of them will build using Sphinx 1.3.1.

Yuri's change appears to be undoing a change he inadvertently committed, which was probably a change he made locally so he could get the docs to build because of this very problem.

David, Thanks for pointing this out. I totally forgot that the 2 and 3 versions use different themes :(

Which version of Sphinx are you using?  The Docs buildbot is happily building with 1.3.1 (though failing the lint check, which I missed when it started).

1.3.1.  The doc build doesn't *fail*, it's just that if you try to view the built pages they don't render correctly, because there is no default.css that the pydoctheme depends on.

I wonder what the web site is using for the build.  Although copying default.css from an old build works (as would, presumably, copying classic.css to default.css or hardlinking it).

Ah, sorry; I missed that distinction.

I can't reproduce, though: in a clean checkout of default, I just did:

cd Doc/
make venv PYTHON=python3
source venv/bin/activate
make html

The resulting build did emit the warning about the themes, but the docs produced render just fine.

Hmm.  All I do is 'make clean; make html', but I don't see why that would make a difference to the problem on point.  

I didn't know about this make venv thing...that must be newish?  When I try it, that does indeed work.  Building that way produces a 'default.css' in build/html/_static, whereas building without the venv (but with sphinx 1.3.1) does not.  I have no idea why, it doesn't really make sense that that would be the case.

Perhaps the experiment of using the installed sphinx to build the docs is a failure and we should just make using local copies the default again?

Please keep the doc build compatible with Sphinx 1.2.*. Sphinx 1.3.1 has a problem with LaTeX generation, so we can't upgrade the automated docs build to use it until 1.3.2 is released.

We had the same problem with PyMongo's docs. The issue happens with Sphinx 1.3.0 and 1.3.1. It appears to be resolved in 1.3.2. The changelog includes this line (no issue number is mentioned):

Add a “default.css” stylesheet (which imports “classic.css”) for compatibility.

http://www.sphinx-doc.org/en/stable/changes.html#release-1-3-2-released-nov-29-2015

After upgrading from 1.3.1 to 1.3.4, I no longer get the warning. Perhaps we can close this now?

We build happily with 1.3.3 on docs.python.org, so looks like everything is okay.