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Sphinx extension for substituting variables

Project description

Sphinx (and Docutils) provide a way to template variables into your documentation, but it can be kind of limited. This Sphinx extension provides a powerful way to manage a master set of documentation with local-specific variations. Logical management is a key design consideration: it’s easy to make some variable substitutions, but harder to keep them up to date when you make a long-running fork!

Features include:

  • Default values can also be included inline and replaced only if a replacement value is defined.

  • Search paths for variable replacements: hierarchical variable substitutions, group and then local customizations supported.

  • Multiple compilation mode: original, with substitutions, or both. In the “both” mode, both the original and substituted are shown with highlighting, so that you can easily compare the current original values with your current substitute values.

  • Both role (inline) and directive (paragraph level) support.

Usage examples

As a role:

SSH to :sub:`(hostname)`

We see the ReST role is named sub (which conflicts with “subscript”, maybe we should change that) and an ID of hostname. If there is a file called hostname.rst in the search path or a file with a .yaml extension with a hostname key in the search path, it will substitute that value into this place. If this value doesn’t exist, it will use the original (or remove it if the original is empty).

As a directive:

.. sub:: ssh_clarification

This directive works analogously to the role, with an ID of ssh_clarification. There is no default content in this example, which means that nothing is inserted unless a replacement is defined , so in this case when used without content it can be used to add in an extra content in just some versions. (There could be default content, too.) If the directive had content, then it would work the same as the first example.

As a reminder, in Docutils/Sphinx, a role is inline text and a directive is a paragraph-level structure.

There are three run modes, controlled by the substitution_mode configuration option:

  • replace: Use the replacements if defined or else the original, with no special markup. This is the default mode and does what you expect for normal use.

  • both: Show both the ID, original value, and the replacement value with distinguishing markup (HTML only). This can be used to compare your local version with the latest upstream. to see what needs to be updated - or what substitutions are available and should be used.

  • original: Show only the original text without any replacements.

Finally, there is a sub-list directive:

.. sub-list::

This directive is replaced with a table that contains all substitution IDs, original values, and replacements that have been used in the document.

When searching for replacement values:

  1. First, each item in the search path (configuration option substitute_path) is searched.

    1. If it’s a file, load it as YAML (see below)

    2. If it’s a directory, go to step 2.

  2. List the directory and search for files to load.

    1. First, load all *.rst files. The * value is used as the replacement ID.

    2. Second, search for all *.yaml files. Load them as YAML data, which should be a mapping from keys to values.

    3. All values are .strip()ped.

  3. Use the first-detected value for each ID. Thus, earlier items in the search path take precedence over later ones.

YAML reminder:

ID1: this is the text for replacement id = ID1
ID2: |
    This is a block text that preserves newlines.

    The "|" character is what indicates that newlines should be
ID3: >
    Using the ">" character removes all newlines and runs the block
    text together.


Install the extension. Currently not in any package managers, so:

pip install

Add the extension to your Sphinx file:

extensions = [

There are no non-trivial dependencies besides PyYAML (which is listed as a dependency, but if you don’t use the YAML feature it isn’t needed). Sphinx and Docutils are obviously required - our goal is to support any reasonable version.


Currently there are two Sphinx variables defined:

  • substitute_mode: One of replace (the default), original, or both. See above for the meaning of these values.

  • substitute_path is a path to search for replacement variables, keyed by ID. In Sphinx, this is a list of paths, but if given on command with -D substitute_path=dir1:dir2, you can colon-separate paths as well. Each file on this path that ends in .rst or .yaml is searched for variables. The default is ..

  • The environment variable SPHINX_EXT_SUBSTITUTION_PATH is used before the substitute_path configuration option. Both are used if both are given (the env var takes precedence). There is no need for both, but it provides more flexible configuration for integration to your build system.

Development and maintenance

Most functonality exists and this is now usable, but not extensively used yet. Please send any changes or requests to us. This was developed as a first non-trivial Sphinx extension, so any refactorings to make things better are welcome.

Primary maintainer: Richard Darst, Aalto University.

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