.. _dev-extensions:

Developing extensions for Sphinx
================================

Since many projects will need special features in their documentation, Sphinx is
designed to be extensible on several levels.

This is what you can do in an extension: First, you can add new
:term:`builder`\s to support new output formats or actions on the parsed
documents.  Then, it is possible to register custom reStructuredText roles and
directives, extending the markup.  And finally, there are so-called "hook
points" at strategic places throughout the build process, where an extension can
register a hook and run specialized code.

An extension is simply a Python module.  When an extension is loaded, Sphinx
imports this module and executes its ``setup()`` function, which in turn
notifies Sphinx of everything the extension offers -- see the extension tutorial
for examples.

The configuration file itself can be treated as an extension if it contains a
``setup()`` function.  All other extensions to load must be listed in the
:confval:`extensions` configuration value.

Extension metadata
------------------

.. versionadded:: 1.3

The ``setup()`` function can return a dictionary.  This is treated by Sphinx
as metadata of the extension.  Metadata keys currently recognized are:

* ``'version'``: a string that identifies the extension version.  It is used for
  extension version requirement checking (see :confval:`needs_extensions`) and
  informational purposes.  If not given, ``"unknown version"`` is substituted.
* ``'parallel_read_safe'``: a boolean that specifies if parallel reading of
  source files can be used when the extension is loaded.  It defaults to
  ``False``, i.e. you have to explicitly specify your extension to be
  parallel-read-safe after checking that it is.
* ``'parallel_write_safe'``: a boolean that specifies if parallel writing of
  output files can be used when the extension is loaded.  Since extensions
  usually don't negatively influence the process, this defaults to ``True``.

APIs used for writing extensions
--------------------------------

.. toctree::

   tutorial
   appapi
   envapi
   builderapi
   markupapi
   domainapi
   parserapi
   nodes
