Creating reStructuredText Directives

The Directive Class

Directives are created by defining a directive class that inherits from docutils.parsers.rst.Directive:

from docutils.parsers import rst

class MyDirective(rst.Directive):

    ...

To understand how to implement the directive, let's have a look at the docstring of the Directive base class:

>>> from docutils.parsers import rst
>>> print rst.Directive.__doc__

    Base class for reStructuredText directives.

    The following attributes may be set by subclasses.  They are
    interpreted by the directive parser (which runs the directive
    class):

    - `required_arguments`: The number of required arguments (default:
      0).

    - `optional_arguments`: The number of optional arguments (default:
      0).

    - `final_argument_whitespace`: A boolean, indicating if the final
      argument may contain whitespace (default: False).

    - `option_spec`: A dictionary, mapping known option names to
      conversion functions such as `int` or `float` (default: {}, no
      options).  Several conversion functions are defined in the
      directives/__init__.py module.

      Option conversion functions take a single parameter, the option
      argument (a string or ``None``), validate it and/or convert it
      to the appropriate form.  Conversion functions may raise
      `ValueError` and `TypeError` exceptions.

    - `has_content`: A boolean; True if content is allowed.  Client
      code must handle the case where content is required but not
      supplied (an empty content list will be supplied).

    Arguments are normally single whitespace-separated words.  The
    final argument may contain whitespace and/or newlines if
    `final_argument_whitespace` is True.

    If the form of the arguments is more complex, specify only one
    argument (either required or optional) and set
    `final_argument_whitespace` to True; the client code must do any
    context-sensitive parsing.

    When a directive implementation is being run, the directive class
    is instantiated, and the `run()` method is executed.  During
    instantiation, the following instance variables are set:

    - ``name`` is the directive type or name (string).

    - ``arguments`` is the list of positional arguments (strings).

    - ``options`` is a dictionary mapping option names (strings) to
      values (type depends on option conversion functions; see
      `option_spec` above).

    - ``content`` is a list of strings, the directive content line by line.

    - ``lineno`` is the line number of the first line of the directive.

    - ``content_offset`` is the line offset of the first line of the content from
      the beginning of the current input.  Used when initiating a nested parse.

    - ``block_text`` is a string containing the entire directive.

    - ``state`` is the state which called the directive function.

    - ``state_machine`` is the state machine which controls the state which called
      the directive function.

    Directive functions return a list of nodes which will be inserted
    into the document tree at the point where the directive was
    encountered.  This can be an empty list if there is nothing to
    insert.

    For ordinary directives, the list must contain body elements or
    structural elements.  Some directives are intended specifically
    for substitution definitions, and must return a list of `Text`
    nodes and/or inline elements (suitable for inline insertion, in
    place of the substitution reference).  Such directives must verify
    substitution definition context, typically using code like this::

        if not isinstance(state, states.SubstitutionDef):
            error = state_machine.reporter.error(
                'Invalid context: the "%s" directive can only be used '
                'within a substitution definition.' % (name),
                nodes.literal_block(block_text, block_text), line=lineno)
            return [error]

>>>

Option Conversion Functions

An option specification (Directive.option_spec) must be defined detailing the options available to the directive. An option spec is a mapping of option name to conversion function; conversion functions are applied to each option value to check validity and convert them to the expected type. Python's built-in conversion functions are often usable for this, such as int, float. Other useful conversion functions are included in the docutils.parsers.rst.directives package (in the __init__.py module):

• flag: For options with no option arguments. Checks for an argument (raises ValueError if found), returns None for valid flag options.

• unchanged_required: Returns the text argument, unchanged. Raises ValueError if no argument is found.

• unchanged: Returns the text argument, unchanged. Returns an empty string ("") if no argument is found.

• path: Returns the path argument unwrapped (with newlines removed). Raises ValueError if no argument is found.

• uri: Returns the value (URI-reference) with whitespace removed. Raises ValueError if no argument is found.

• nonnegative_int: Checks for a nonnegative integer argument, and raises ValueError if not.

• class_option: Converts the argument into an ID-compatible string and returns it. Raises ValueError if no argument is found.

• unicode_code: Convert a Unicode character code to a Unicode character.

• single_char_or_unicode: A single character is returned as-is. Unicode characters codes are converted as in unicode_code.

• single_char_or_whitespace_or_unicode: As with single_char_or_unicode, but "tab" and "space" are also supported.

• positive_int: Converts the argument into an integer. Raises ValueError for negative, zero, or non-integer values.

• positive_int_list: Converts a space- or comma-separated list of integers into a Python list of integers. Raises ValueError for non-positive-integer values.

• encoding: Verifies the encoding argument by lookup. Raises ValueError for unknown encodings.

A further utility function, choice, is supplied to enable options whose argument must be a member of a finite set of possible values. A custom conversion function must be written to use it. For example:

from docutils.parsers.rst import directives

def yesno(argument):
    return directives.choice(argument, ('yes', 'no'))

For example, here is an option spec for a directive which allows two options, "name" and "value", each with an option argument:

option_spec = {'name': unchanged, 'value': int}

Error Handling

If your directive implementation encounters an error during processing, you should call self.error() inside the run() method:

if error_condition:
    raise self.error('Error message.')

The self.error() method will immediately raise an exception that will be caught by the reStructuredText directive handler. The directive handler will then insert an error-level system message in the document at the place where the directive occurred.

Instead of self.error, you can also use self.severe and self.warning for more or less severe problems.

If you want to return a system message and document contents, you need to create the system message yourself instead of using the self.error convenience method:

def run(self):
    # Create node(s).
    node = nodes.paragraph(...)
    # Node list to return.
    node_list = [node]
    if error_condition:
         # Create system message.
         error = self.reporter.error(
             'Error in "%s" directive: Your error message.' % self.name,
             nodes.literal_block(block_text, block_text), line=lineno)
         node_list.append(error)
    return node_list

Register the Directive

• If the directive is a general-use addition to the Docutils core, it must be registered with the parser and language mappings added:

  1. Register the new directive using its canonical name in docutils/parsers/rst/directives/__init__.py, in the _directive_registry dictionary. This allows the reStructuredText parser to find and use the directive.

  2. Add an entry to the directives dictionary in docutils/parsers/rst/languages/en.py for the directive, mapping the English name to the canonical name (both lowercase). Usually the English name and the canonical name are the same.

  3. Update all the other language modules as well. For languages in which you are proficient, please add translations. For other languages, add the English directive name plus "(translation required)".

• If the directive is application-specific, use the register_directive function:

  from docutils.parsers.rst import directives
  directives.register_directive(directive_name, directive_class)

Examples

For the most direct and accurate information, "Use the Source, Luke!". All standard directives are documented in reStructuredText Directives, and the source code implementing them is located in the docutils/parsers/rst/directives package. The __init__.py module contains a mapping of directive name to module and function name. Several representative directives are described below.

class Note(BaseAdmonition):

    node_class = nodes.note

# Import Docutils document tree nodes module.
from docutils import nodes
# Import Directive base class.
from docutils.parsers.rst import Directive

class BaseAdmonition(Directive):

    required_arguments = 0
    optional_arguments = 0
    final_argument_whitespace = True
    option_spec = {}
    has_content = True

    node_class = None
    """Subclasses must set this to the appropriate admonition node class."""

    def run(self):
        # Raise an error if the directive does not have contents.
        self.assert_has_content()
        text = '\n'.join(self.content)
        # Create the admonition node, to be populated by `nested_parse`.
        admonition_node = self.node_class(rawsource=text)
        # Parse the directive contents.
        self.state.nested_parse(self.content, self.content_offset,
                                admonition_node)
        return [admonition_node]

# Import Docutils document tree nodes module.
from docutils import nodes
# Import ``directives`` module (contains conversion functions).
from docutils.parsers.rst import directives
# Import Directive base class.
from docutils.parsers.rst import Directive

def align(argument):
    """Conversion function for the "align" option."""
    return directives.choice(argument, ('left', 'center', 'right'))

class Image(Directive):

    required_arguments = 1
    optional_arguments = 0
    final_argument_whitespace = True
    option_spec = {'alt': directives.unchanged,
                   'height': directives.nonnegative_int,
                   'width': directives.nonnegative_int,
                   'scale': directives.nonnegative_int,
                   'align': align,
                   }
    has_content = False

    def run(self):
        reference = directives.uri(self.arguments[0])
        self.options['uri'] = reference
        image_node = nodes.image(rawsource=self.block_text,
                                 **self.options)
        return [image_node]