Creating New Domains

In the event a new type of object or reference needs to be documented, and none of the existing markup options or domains are appropriate, it is possible to extend reStructuredText and Sphinx by adding custom domains.

Each domain may be designed to accept any number of required and optional arguments, as well as any collection of domain options, and each option may be designed to support arbitrary values, restricted (enumerated) values, or to simply act as flags.

All custom domain definitions should be located in doc/ext/ and consist of, at a bare minimum, a domain class definition and a domain reference class definition. Sphinx domains are implemented in Python.

For this section, we will use the contrived example of creating a domain which permits us to define and reference a set of variables which are constrained by the following characteristics:

  1. Each variable in the domain must be one of known list of data types, which we will limit here to the possibilities of integer, float, string.

  2. Where the data type is not specified, we can assume it is string.

  3. Variables which are numeric in their type may have a range of permissible values.

  4. Variables in the domain may still be present and supported in the system, but are planned to be removed in some future release.

  5. Every variable is associated with a single URI protocol, though there is no validation performed on the value used to represent the protocol name.

As stated, this example is fairly contrived and would not match any particularly likely real-world needs, but it will allow us to demonstrate the full extent of custom domain definition without needless complexity, reader’s suspension of disbelief permitting.

For this chapter’s purpose, we will call this domain simply Variables, and we will construct classes which allow us to document variables thusly:

.. ts:variable:: http_enabled http integer

   Enables (any positive, non-zero value) or disables (any zero or negative
   value) processing of HTTP requests.

And referencing of those variables defined with this domain via:


Defining the Domain

Each domain is defined by a class which inherits from std.Target. Several class attributes are expected, which determine how domain object definitions are processed. Traffic Server convention is to name each domain’s class in camel case, beginning with TS to prevent any class name collisions with builtin Sphinx classes.

class TSVariable(std.Target):

We have named the domain’s defining class as TSVariable and inherited from the std.Target class. Given the earlier stated requirements, we need a domain which supports at least two required attributes (a name, of course, and a URI protocol with which it is associated) and a commonly defined, though optional, third attribute (a data type). We’ll deal with the value ranges and deprecation status later.

class TSVariable(std.Target):
    required_arguments = 2
    optional_arguments = 1
    final_argument_whitespace = False
    has_content = True

We’ve now specified the appropriate number of required and optional arguments, though not what each one happens to be or in what order the required arguments need be written. Additionally, we’ve declared that definitions using this domain do not permit whitespace in the final argument, but definitions can have a block of text content which follows them and should be associated with the item being defined.


Permitting whitespace in the final argument causes the final value of a valid definition to slurp the remaining content of the definition. Normally, each argument is separated by whitespace, thus foo bar baz would only be a valid definition if the domain’s required and optional argument counts added up to exactly three. If the domain defined only two arguments as expected, but sets final_argument_whitespace to True, then the definition would be valid and the second argument in this case would be bar baz.

Our requirements also state support for optional value ranges, and a flag to indicate whether the variable is being deprecated. These can easily be supported through the option_spec, which allows for options to be tagged on to a domain item, on the lines immediately following its definition.

class TSVariable(std.Target):
    option_spec = {
        'deprecated' : rst.directives.flag,
        'range' : rst.directives.unchanged

For our example, deprecated is simply a boolean flag, and range will be an arbitrary string on which we will perform no particular transformation or validation (good behavior will be left up to those documenting their variables with this domain). The rst.directives module may be consulted for a wider range of predefined option types, including the ability to define your own types which can perform any complexity of validation you may desire to implement.

It would be good form to also include a docstring for the class explaining the expected arguments in brief. With that included, our class now looks like:

class TSVariable(std.Target):
    Description of a Traffic Server protocol variable.

    Required arguments, in order, are:

        URI Protocol
        Variable name

    Optional argument is the data type of the variable, with "string" the
    the default. Possible values are: "string", "integer", and "float".

    Options supported are:

        :deprecated: - A simple flag option indicating whether the variable
        is slated for removal in future releases.

        :range: - A string describing the permissible range of values the
        variable may contain.

    option_spec = {
        'deprecated' : rst.directives.flag,
        'range' : rst.directives.unchanged

    required_arguments = 2
    optional_arguments = 1
    final_argument_whitespace = False
    has_content = True

Every domain class must also provide a run method, which is called every time an item definition using the domain is encountered. This method is where all argument and option validations are performed, and where transformation of the definition into the documentation’s rendered output occurs.

The core responsibilities of the run method in a domain class are to populate the domain’s data dictionary, for use by references, as well as to transform the item’s definition into a document structure suitable for rendering. The default title to be used for references will be constructed in this method, and all arguments and options will be processed.

Our variables domain might have the following run method:

def run(self):
    var_name, var_proto = self.arguments[0:2]
    var_type = 'string'

    if (len(self.arguments) > 2):
        var_type = self.arguments[2]

    # Create a documentation node to use as the parent.
    node = sphinx.addnodes.desc()
    node.document = self.state.document
    node['objtype'] = 'variable'

    # Add the signature child node for permalinks.
    title = sphinx.addnodes.desc_signature(var_name, '')
    title['first'] = False
    title['objtype'] = node['objtype']
    title['classes'] = 'ts-variable-title'

    title += sphinx.addnodes.desc_name(var_name, var_name)

    env.domaindata['ts']['variable'][var_name] = env.docname

    # Create table detailing all provided domain options
    fl = nodes.field_list()

    if ('deprecated' in self.options):
        fl.append(self.make_field('Deprecated', 'Yes'))

    if ('range' in self.options):
        fl.append(self.make_field('Value range:', self.options['range']))

    # Parse any associated block content for the item's description
    nn = nodes.compound()
    self.state.nested_parse(self.content, self.content_offset, nn)

    # Create an index node so Sphinx will list this variable and its
    # references in the index section.
    indexnode = sphinx.addnodes.index(entries=[])
        ('single', _('%s') % var_name, nodes.make_id(var_name), '')

    return [ indexnode, node, fl, nn ]

Defining the Domain Reference

Domain reference definitions are quite simple in comparison to the full domain definition. As with the domain itself, they are defined by a single class, but inherit from XRefRole instead. There are no attributes necessary, and only a single method, process_link need be defined.

For our variables domain references, the class definition is a very short one. Traffic Server convention is to name the reference class the same as the domain class, but with Ref appended to the name. Thus, the domain class TSVariable is accompanied by a TSVariableRef reference class.

class TSVariableRef(XRefRole):
    def process_link(self, env, ref_node, explicit_title_p, title, target):
        return title, target

The process_link method will receive several arguments, as described below, and should return two values: a string containing the title of the reference, and a hyperlink target to be used for the rendered documentation.

The process_link method receives the following arguments:


The reference instance object, as per Python method conventions.


A dictionary object containing the environment of the documentation processor in its state at the time of the reference encounter.


The node object of the reference as encountered in the documentation source.


Contains the text content of the reference’s explicit title overriding, if present in the reference markup.


The processed form of the reference title, which may be the result of domain class transformations or an overriding of the reference title within the reference itself.


The computed target of the reference, suitable for use by Sphinx to construct hyperlinks to the location of the item’s definition, wherever it may reside in the final rendered form of the documentation.

In our reference class, we have simply returned the processed title (allowing the documentation to override the variable’s name if desired, or defaulting to the domain class’s representation of the variable name in all other cases) and the parser’s computed target.

It is recommended to leave the target untouched, however you may choose to perform any transformations you wish on the value of the title, bearing in mind that whatever string is returned will appear verbatim in the rendered documentation everywhere references for this domain are used.

Exporting the Domain

With both the domain itself and references to it now defined, the final step is to register those classes as domain and reference handlers in a namespace. This is done for Traffic Server (in its :ts: namespace) quite easily by modifying the TrafficServerDomain class, also located in doc/ext/

The following dictionaries defined by that class should be updated to include the new domain and reference. In each case, the key used when adding to the dictionary should be the string you wish to use in documentation markup for your new domain. In our example’s case, we will choose variable since it aligns with the Python classes we’ve created above, and their contrived purpose.


Used to define the actual markup string


Defines which class is used to implement a given domain.


Defines the class used to implement references to a domain.


Used to initialized the dictionary which tracks all encountered instances of each domain. This should always be set to an empty dictionary for each domain.


May be used to provide a default warning if a reference is attempted to a non-existent item for a domain.