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diff --git a/doc/extdev/tutorial.rst b/doc/extdev/tutorial.rst new file mode 100644 index 00000000..38264190 --- /dev/null +++ b/doc/extdev/tutorial.rst @@ -0,0 +1,397 @@ +.. _exttut: + +Tutorial: Writing a simple extension +==================================== + +This section is intended as a walkthrough for the creation of custom extensions. +It covers the basics of writing and activating an extensions, as well as +commonly used features of extensions. + +As an example, we will cover a "todo" extension that adds capabilities to +include todo entries in the documentation, and collecting these in a central +place. (A similar "todo" extension is distributed with Sphinx.) + + +Important objects +----------------- + +There are several key objects whose API you will use while writing an +extension. These are: + +**Application** + The application object (usually called ``app``) is an instance of + :class:`.Sphinx`. It controls the most high-level functionality, such as the + setup of extensions, event dispatching and producing output (logging). + + If you have the environment object, the application is available as + ``env.app``. + +**Environment** + The build environment object (usually called ``env``) is an instance of + :class:`.BuildEnvironment`. It is responsible for parsing the source + documents stores all metadata about the document collection and is serialized + after each build. + + Its API provides methods to do with access to metadata, resolving references, + etc. It can also be used by extensions to cache information that should + persist for incremental rebuilds. + + If you have the application or builder object, the environment is available + as ``app.env`` or ``builder.env``. + +**Builder** + The builder object (usually called ``builder``) is an instance of a specific + subclass of :class:`.Builder`. Each builder class knows how to convert the + parsed documents into an output format, or otherwise process them (e.g. check + external links). + + If you have the application object, the environment is available as + ``app.builder``. + +**Config** + The config object (usually called ``config``) provides the values of + configuration values set in :file:`conf.py` as attributes. It is an instance + of :class:`.Config`. + + The config is available as ``app.config`` or ``env.config``. + + +Build Phases +------------ + +One thing that is vital in order to understand extension mechanisms is the way +in which a Sphinx project is built: this works in several phases. + +**Phase 0: Initialization** + + In this phase, almost nothing interesting for us happens. The source + directory is searched for source files, and extensions are initialized. + Should a stored build environment exist, it is loaded, otherwise a new one is + created. + +**Phase 1: Reading** + + In Phase 1, all source files (and on subsequent builds, those that are new or + changed) are read and parsed. This is the phase where directives and roles + are encountered by the docutils, and the corresponding code is executed. The + output of this phase is a *doctree* for each source files, that is a tree of + docutils nodes. For document elements that aren't fully known until all + existing files are read, temporary nodes are created. + + There are nodes provided by docutils, which are documented `in the docutils + documentation <http://docutils.sourceforge.net/docs/ref/doctree.html>`__. + Additional nodes are provided by Sphinx and :ref:`documented here <nodes>`. + + During reading, the build environment is updated with all meta- and cross + reference data of the read documents, such as labels, the names of headings, + described Python objects and index entries. This will later be used to + replace the temporary nodes. + + The parsed doctrees are stored on the disk, because it is not possible to + hold all of them in memory. + +**Phase 2: Consistency checks** + + Some checking is done to ensure no surprises in the built documents. + +**Phase 3: Resolving** + + Now that the metadata and cross-reference data of all existing documents is + known, all temporary nodes are replaced by nodes that can be converted into + output. For example, links are created for object references that exist, and + simple literal nodes are created for those that don't. + +**Phase 4: Writing** + + This phase converts the resolved doctrees to the desired output format, such + as HTML or LaTeX. This happens via a so-called docutils writer that visits + the individual nodes of each doctree and produces some output in the process. + +.. note:: + + Some builders deviate from this general build plan, for example, the builder + that checks external links does not need anything more than the parsed + doctrees and therefore does not have phases 2--4. + + +Extension Design +---------------- + +We want the extension to add the following to Sphinx: + +* A "todo" directive, containing some content that is marked with "TODO", and + only shown in the output if a new config value is set. (Todo entries should + not be in the output by default.) + +* A "todolist" directive that creates a list of all todo entries throughout the + documentation. + +For that, we will need to add the following elements to Sphinx: + +* New directives, called ``todo`` and ``todolist``. +* New document tree nodes to represent these directives, conventionally also + called ``todo`` and ``todolist``. We wouldn't need new nodes if the new + directives only produced some content representable by existing nodes. +* A new config value ``todo_include_todos`` (config value names should start + with the extension name, in order to stay unique) that controls whether todo + entries make it into the output. +* New event handlers: one for the :event:`doctree-resolved` event, to replace + the todo and todolist nodes, and one for :event:`env-purge-doc` (the reason + for that will be covered later). + + +The Setup Function +------------------ + +.. currentmodule:: sphinx.application + +The new elements are added in the extension's setup function. Let us create a +new Python module called :file:`todo.py` and add the setup function:: + + def setup(app): + app.add_config_value('todo_include_todos', False, False) + + app.add_node(todolist) + app.add_node(todo, + html=(visit_todo_node, depart_todo_node), + latex=(visit_todo_node, depart_todo_node), + text=(visit_todo_node, depart_todo_node)) + + app.add_directive('todo', TodoDirective) + app.add_directive('todolist', TodolistDirective) + app.connect('doctree-resolved', process_todo_nodes) + app.connect('env-purge-doc', purge_todos) + +The calls in this function refer to classes and functions not yet written. What +the individual calls do is the following: + +* :meth:`~Sphinx.add_config_value` lets Sphinx know that it should recognize the + new *config value* ``todo_include_todos``, whose default value should be + ``False`` (this also tells Sphinx that it is a boolean value). + + If the third argument was ``True``, all documents would be re-read if the + config value changed its value. This is needed for config values that + influence reading (build phase 1). + +* :meth:`~Sphinx.add_node` adds a new *node class* to the build system. It also + can specify visitor functions for each supported output format. These visitor + functions are needed when the new nodes stay until phase 4 -- since the + ``todolist`` node is always replaced in phase 3, it doesn't need any. + + We need to create the two node classes ``todo`` and ``todolist`` later. + +* :meth:`~Sphinx.add_directive` adds a new *directive*, given by name and class. + + The handler functions are created later. + +* Finally, :meth:`~Sphinx.connect` adds an *event handler* to the event whose + name is given by the first argument. The event handler function is called + with several arguments which are documented with the event. + + +The Node Classes +---------------- + +Let's start with the node classes:: + + from docutils import nodes + + class todo(nodes.Admonition, nodes.Element): + pass + + class todolist(nodes.General, nodes.Element): + pass + + def visit_todo_node(self, node): + self.visit_admonition(node) + + def depart_todo_node(self, node): + self.depart_admonition(node) + +Node classes usually don't have to do anything except inherit from the standard +docutils classes defined in :mod:`docutils.nodes`. ``todo`` inherits from +``Admonition`` because it should be handled like a note or warning, ``todolist`` +is just a "general" node. + +.. note:: + + Many extensions will not have to create their own node classes and work fine + with the nodes already provided by `docutils + <http://docutils.sourceforge.net/docs/ref/doctree.html>`__ and :ref:`Sphinx + <nodes>`. + + +The Directive Classes +--------------------- + +A directive class is a class deriving usually from +``docutils.parsers.rst.Directive``. The directive interface is covered in +detail in the `docutils documentation`_; the important thing is that the class +has attributes that configure the allowed markup and a method ``run`` that +returns a list of nodes. + +The ``todolist`` directive is quite simple:: + + from docutils.parsers.rst import Directive + + class TodolistDirective(Directive): + + def run(self): + return [todolist('')] + +An instance of our ``todolist`` node class is created and returned. The +todolist directive has neither content nor arguments that need to be handled. + +The ``todo`` directive function looks like this:: + + from sphinx.util.compat import make_admonition + + class TodoDirective(Directive): + + # this enables content in the directive + has_content = True + + def run(self): + env = self.state.document.settings.env + + targetid = "todo-%d" % env.new_serialno('todo') + targetnode = nodes.target('', '', ids=[targetid]) + + ad = make_admonition(todo, self.name, [_('Todo')], self.options, + self.content, self.lineno, self.content_offset, + self.block_text, self.state, self.state_machine) + + if not hasattr(env, 'todo_all_todos'): + env.todo_all_todos = [] + env.todo_all_todos.append({ + 'docname': env.docname, + 'lineno': self.lineno, + 'todo': ad[0].deepcopy(), + 'target': targetnode, + }) + + return [targetnode] + ad + +Several important things are covered here. First, as you can see, you can refer +to the build environment instance using ``self.state.document.settings.env``. + +Then, to act as a link target (from the todolist), the todo directive needs to +return a target node in addition to the todo node. The target ID (in HTML, this +will be the anchor name) is generated by using ``env.new_serialno`` which +returns a new unique integer on each call and therefore leads to unique target +names. The target node is instantiated without any text (the first two +arguments). + +An admonition is created using a standard docutils function (wrapped in Sphinx +for docutils cross-version compatibility). The first argument gives the node +class, in our case ``todo``. The third argument gives the admonition title (use +``arguments`` here to let the user specify the title). A list of nodes is +returned from ``make_admonition``. + +Then, the todo node is added to the environment. This is needed to be able to +create a list of all todo entries throughout the documentation, in the place +where the author puts a ``todolist`` directive. For this case, the environment +attribute ``todo_all_todos`` is used (again, the name should be unique, so it is +prefixed by the extension name). It does not exist when a new environment is +created, so the directive must check and create it if necessary. Various +information about the todo entry's location are stored along with a copy of the +node. + +In the last line, the nodes that should be put into the doctree are returned: +the target node and the admonition node. + +The node structure that the directive returns looks like this:: + + +--------------------+ + | target node | + +--------------------+ + +--------------------+ + | todo node | + +--------------------+ + \__+--------------------+ + | admonition title | + +--------------------+ + | paragraph | + +--------------------+ + | ... | + +--------------------+ + + +The Event Handlers +------------------ + +Finally, let's look at the event handlers. First, the one for the +:event:`env-purge-doc` event:: + + def purge_todos(app, env, docname): + if not hasattr(env, 'todo_all_todos'): + return + env.todo_all_todos = [todo for todo in env.todo_all_todos + if todo['docname'] != docname] + +Since we store information from source files in the environment, which is +persistent, it may become out of date when the source file changes. Therefore, +before each source file is read, the environment's records of it are cleared, +and the :event:`env-purge-doc` event gives extensions a chance to do the same. +Here we clear out all todos whose docname matches the given one from the +``todo_all_todos`` list. If there are todos left in the document, they will be +added again during parsing. + +The other handler belongs to the :event:`doctree-resolved` event. This event is +emitted at the end of phase 3 and allows custom resolving to be done:: + + def process_todo_nodes(app, doctree, fromdocname): + if not app.config.todo_include_todos: + for node in doctree.traverse(todo): + node.parent.remove(node) + + # Replace all todolist nodes with a list of the collected todos. + # Augment each todo with a backlink to the original location. + env = app.builder.env + + for node in doctree.traverse(todolist): + if not app.config.todo_include_todos: + node.replace_self([]) + continue + + content = [] + + for todo_info in env.todo_all_todos: + para = nodes.paragraph() + filename = env.doc2path(todo_info['docname'], base=None) + description = ( + _('(The original entry is located in %s, line %d and can be found ') % + (filename, todo_info['lineno'])) + para += nodes.Text(description, description) + + # Create a reference + newnode = nodes.reference('', '') + innernode = nodes.emphasis(_('here'), _('here')) + newnode['refdocname'] = todo_info['docname'] + newnode['refuri'] = app.builder.get_relative_uri( + fromdocname, todo_info['docname']) + newnode['refuri'] += '#' + todo_info['target']['refid'] + newnode.append(innernode) + para += newnode + para += nodes.Text('.)', '.)') + + # Insert into the todolist + content.append(todo_info['todo']) + content.append(para) + + node.replace_self(content) + +It is a bit more involved. If our new "todo_include_todos" config value is +false, all todo and todolist nodes are removed from the documents. + +If not, todo nodes just stay where and how they are. Todolist nodes are +replaced by a list of todo entries, complete with backlinks to the location +where they come from. The list items are composed of the nodes from the todo +entry and docutils nodes created on the fly: a paragraph for each entry, +containing text that gives the location, and a link (reference node containing +an italic node) with the backreference. The reference URI is built by +``app.builder.get_relative_uri`` which creates a suitable URI depending on the +used builder, and appending the todo node's (the target's) ID as the anchor +name. + +.. _docutils documentation: http://docutils.sourceforge.net/docs/ref/rst/directives.html |