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Python Enhancement Proposals

An Overview of the PEP Rendering System

This document provides an overview of the PEP rendering system, as a companion to PEP 676.

1. Configuration

Configuration is stored in three files:

  • conf.py contains the majority of the Sphinx configuration
  • contents.rst creates the Sphinx-mandated table of contents directive
  • pep_sphinx_extensions/pep_theme/theme.conf sets the Pygments themes

The configuration:

  • registers the custom Sphinx extension
  • sets both .txt and .rst suffixes to be parsed as PEPs
  • tells Sphinx which source files to use
  • registers the PEP theme, maths renderer, and template
  • disables some default settings that are covered in the extension
  • sets the default and “dark mode” code formatter styles

2. Orchestration

build.py manages the rendering process. Usage is covered in Building PEPs Locally.

3. Extension

The Sphinx extension and theme are contained in the pep_sphinx_extensions directory. The following is a brief overview of the stages of the PEP rendering process, and how the extension functions at each point.

3.1 Extension setup

The extension registers several objects:

  • FileBuilder and DirectoryBuilder run the build process for file- and directory-based building, respectively.
  • PEPParser registers the custom document transforms and parses PEPs to a Docutils document.
  • PEPTranslator converts a Docutils document into HTML.
  • PEPRole handles :pep: roles in the reStructuredText source.

The extension also patches default behaviour:

  • updating the default settings
  • updating the Docutils inliner
  • using HTML maths display over MathJax

3.2 Builder initialised

After the Sphinx builder object is created and initialised, we ensure the configuration is correct for the builder chosen.

Currently this involves updating the relative link template. See _update_config_for_builder in pep_sphinx_extensions/__init__.py.

3.3 Before documents are read

The create_pep_zero hook is called. See 5. PEP 0.

3.4 Read document

Parsing the document is handled by PEPParser (pep_sphinx_extensions.pep_processor.parsing.pep_parser.PEPParser), a lightweight wrapper over sphinx.parsers.RSTParser.

PEPParser reads the document with leading RFC 2822 headers and registers the transforms we want to apply. These are:

  • PEPHeaders
  • PEPTitle
  • PEPContents
  • PEPFooter

Transforms are then applied in priority order.

3.4.1 PEPRole role

This overrides the built-in :pep: role to return the correct URL.

3.4.2 PEPHeaders transform

PEPs start with a set of RFC 2822 headers, per PEP 1. This transform validates that the required headers are present and of the correct data type, and removes headers not for display. It must run before the PEPTitle transform.

3.4.3 PEPTitle transform

We generate the title node from the parsed title in the PEP headers, and make all nodes in the document children of the new title node. This transform must also handle parsing reStructuredText markup within PEP titles, such as PEP 604.

3.4.4 PEPContents transform

The automatic table of contents (TOC) is inserted in this transform in a two-part process.

First, the transform inserts a placeholder for the TOC and a horizontal rule after the document title and PEP headers. A callback transform then recursively walks the document to create the TOC, starting from after the placeholder node. Whilst walking the document, all reference nodes in the titles are removed, and titles are given a self-link.

3.4.5 PEPFooter transform

This first builds a map of file modification times from a single git call, as a speed-up. This will return incorrect results on a shallow checkout of the repository, as is the default on continuous integration systems.

We then attempt to remove any empty references sections, and append metadata in the footer (source link and last modified timestamp).

3.5 Prepare for writing

pep_html_builder.FileBuilder.prepare_writing initialises the bare miniumum of the Docutils writer and the settings for writing documents. This provides a significant speed-up over the base Sphinx implementation, as most of the data automatically initialised was unused.

3.6 Translate Docutils to HTML

PEPTranslator overrides paragraph and reference logic to replicate processing from the previous docutils.writers.pep-based system. Paragraphs are made compact where possible by omitting <p> tags, and footnote references are be enclosed in square brackets.

3.7 Prepare for export to Jinja

Finally in pep_html_builder, we gather all the parts to be passed to the Jinja template. This is also where we create the sidebar table of contents.

The HTML files are then written out to the build directory.

4. Theme

The theme is comprised of the HTML template in pep_sphinx_extensions/pep_theme/templates/page.html and the stylesheets in pep_sphinx_extensions/pep_theme/static.

The template is entirely self-contained, not relying on any default behaviour from Sphinx. It specifies the CSS files to include, the favicon, and basic semantic information for the document structure.

The styles are defined in two parts:

  • style.css handles the meat of the layout
  • mq.css adds media queries for a responsive design

5. PEP 0

The generation of the index, PEP 0, happens in three phases. The reStructuredText source file is generated, it is then added to Sphinx, and finally the data is post processed.

5.1 File creation

pep-0000.rst is created during a callback, before documents are loaded by Sphinx.

We first parse the individual PEP files to get the RFC 2822 header, and then parse and validate that metadata.

After collecting and validating all the PEP data, the index itself is created in three steps:

  1. Output the header text
  2. Output the category and numerical indices
  3. Output the author index

The AUTHOR_OVERRIDES.csv file can be used to override an author’s name in the PEP 0 output.

We then add the newly created PEP 0 file to two Sphinx variables so that it will be processed as a normal source document.

5.2 Post processing

The PEPHeaders transform schedules the PEP 0 post-processing code. This serves two functions: masking email addresses and linking numeric PEP references to the actual documents.

6. RSS Feed

The RSS feed is created by extracting the header metadata and abstract from the ten most recent PEPs.