LaTeX: \titleformat last argument in sphinx.sty should be bracketed, not braced (and is anyhow not needed)

[jfbu]

Describe the bug
This is a prehistoric bug which gets back as far as I can tell to bd50766

The \titleformat from titlesec has only 5 mandatory arguments, the last one is optional and should be bracketed. But our style file uses:

sphinx/sphinx/texinputs/sphinx.sty

Lines 716 to 729 in be20f17

	% Augment the sectioning commands used to get our own font family in place,
	% and reset some internal data items (\titleformat from titlesec package)
	\titleformat{\section}{\Large\py@HeaderFamily}%
	{\py@TitleColor\thesection}{0.5em}{\py@TitleColor}{\py@NormalColor}
	\titleformat{\subsection}{\large\py@HeaderFamily}%
	{\py@TitleColor\thesubsection}{0.5em}{\py@TitleColor}{\py@NormalColor}
	\titleformat{\subsubsection}{\py@HeaderFamily}%
	{\py@TitleColor\thesubsubsection}{0.5em}{\py@TitleColor}{\py@NormalColor}
	% By default paragraphs (and subsubsections) will not be numbered because
	% sphinxmanual.cls and sphinxhowto.cls set secnumdepth to 2
	\titleformat{\paragraph}{\py@HeaderFamily}%
	{\py@TitleColor\theparagraph}{0.5em}{\py@TitleColor}{\py@NormalColor}
	\titleformat{\subparagraph}{\py@HeaderFamily}%
	{\py@TitleColor\thesubparagraph}{0.5em}{\py@TitleColor}{\py@NormalColor}

As a result the macro \py@NormalColor is executed during the PDF build already during loading of package sphinx.sty. As its definition is \color{black} this causes no harm and got undetected.

I hit against the error while refactoring the LaTeX style files, and accidentally moved definition of \py@NormalColor to after it is used in \titleformat.

I was surprised by an undefined control sequence error on pdf build. Indeed TeX is a macro expansion language so at first I did not understand why \py@NormalColor was expanded (at a time it is undefined). Turns out it should have been

...[\py@NormalColor]

As it is braced, \titleformat leaves it in the token input stream and it gets executed. Which causes no harm when it has a definition such as \color{black} as done in sphinx.sty on an earlier line.

To Reproduce
Well, this does not cause a build error, and as all is internal to sphinx.sty loading one can not force one, but it is wrong nevertheless.

Expected behavior
Use \titleformat according to documentation

We see that we don't need that \py@NormalColor anyhow as it got executed at the wrong time...

• Sphinx version: all...
• Sphinx extensions: [e.g. sphinx.ext.autodoc, recommonmark]
• Extra tools: TeXLive