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diff --git a/docs/writing_extensions.txt b/docs/writing_extensions.txt new file mode 100755 index 0000000..df17698 --- /dev/null +++ b/docs/writing_extensions.txt @@ -0,0 +1,388 @@ +### Overview + +Python-Markdown includes an API for extension writers to plug their own +custom functionality and/or syntax into the parser. There are preprocessors +which allow you to alter the source before it is passed to the parser, +inline patterns which allow you to add, remove or override the syntax of +any inline elements, and postprocessors which allow munging of the +output of the parser before it is returned. + +As the parser builds an [ElementTree][] object which is later rendered +as Unicode text, there are also some helpers provided to make manipulation of +the tree easier. Each part of the API is discussed in its respective +section below. You may find reading the source of some [[Available Extensions]] +helpful as well. For example, the [[Footnotes]] extension uses most of the +features documented here. + +* [Preprocessors][] + * [TextPreprocessors][] + * [Line Preprocessors][] +* [InlinePatterns][] +* [Postprocessors][] + * [ElementTree Postprocessors][] + * [TextProstprocessors][] +* [Working with the ElementTree][] +* [Integrating your code into Markdown][] + * [extendMarkdown][] + * [registerExtension][] + * [Config Settings][] + * [makeExtension][] + +<h3 id="preprocessors">Preprocessors</h3> + +Preprocessors munge the source text before it is passed into the Markdown +core. This is an excellent place to clean up bad syntax, extract things the +parser may otherwise choke on and perhaps even store it for later retrieval. + +There are two types of preprocessors: [TextPreprocessors][] and +[Line Preprocessors][]. + +<h4 id="textpreprocessors">TextPreprocessors</h4> + +TextPreprocessors should inherit from `markdown.TextPreprocessor` and implement +a `run` method with one argument `text`. The `run` method of each +TextPreprocessor will be passed the entire source text as a single Unicode +string and should either return that single Unicode string, or an altered +version of it. + +For example, a simple TextPreprocessor that normalizes newlines [^1] might look +like this: + + class NormalizePreprocessor(markdown.TextPreprocessor): + def run(self, text): + return text.replace("\r\n", "\n").replace("\r", "\n") + +[^1]: It should be noted that Markdown already normalizes newlines. This +example is for illustrative purposes only. + +<h4 id="linepreprocessors">Line Preprocessors</h4> + +Line Preprocessors should inherit from `markdown.Preprocessor` and implement +a `run` method with one argument `lines`. The `run` method of each Line +Preprocessor will be passed the entire source text as a list of Unicode strings. +Each string will contain one line of text. The `run` method should return +either that list, or an altered list of Unicode strings. + +A pseudo example: + + class MyPreprocessor(markdown.Preprocessor): + def run(self, lines): + new_lines = [] + for line in lines: + m = MYREGEX.match(line) + if m: + # do stuff + else: + new_lines.append(line) + return new_lines + +<h3 id="inlinepatterns">Inline Patterns</h3> + +Inline Patterns implement the inline HTML element syntax for Markdown such as +`*emphasis*` or `[links](http://example.com)`. Pattern objects should be +instances of classes that inherit from `markdown.Pattern` or one of its +children. Each pattern object uses a single regular expression and must have +the following methods: + +* `getCompiledRegExp()`: Returns a compiled regular expression. +* `handleMatch(m)`: Accepts a match object and returns an ElementTree +element of a plain Unicode string. + +Note that any regular expression returned by `getCompiledRegExp` must capture +the whole block. Therefore, they should all start with `r'^(.*?)'` and end +with `r'(.*?)!'. When using the default `getCompiledRegExp()` method provided +in the `Pattern` you can pass in a regular expression without that and +`getCompiledRegExp` will wrap your expression for you. This means that the first +group of your match will be `m.group(2)` as `m.group(1)` will match everything +before the pattern. + +For an example, consider this simplified emphasis pattern: + + class EmphasisPattern(markdown.Pattern): + def handleMatch(self, m): + el = markdown.etree.Element('em') + el.text = m.group(3) + return el + +As discussed in [Integrating Your Code Into Markdown][], an instance of this +class will need to be provided to Markdown. That instance would be created +like so: + + # an oversimplified regex + MYPATTERN = r'\*([^*]+)\*' + # pass in pattern and create instance + emphasis = EmphasisPattern(MYPATTERN) + +Actually it would not be necessary to create that pattern (and not just because +a more sophisticated emphasis pattern already exists in Markdown). The fact is, +that example pattern is not very DRY. A pattern for `**strong**` text would +be almost identical, with the exception that it would create a 'strong' element. +Therefore, Markdown provides a number of generic pattern classes that can +provide some common functionality. For example, both emphasis and strong are +implemented with separate instances of the `SimpleTagPettern` listed below. +Feel free to use or extend any of these Pattern classes. + +**Generic Pattern Classes** + +* `SimpleTextPattern(pattern)`: + + Returns simple text of `group(2)` of a `pattern`. + +* `SimpleTagPattern(pattern, tag)`: + + Returns an element of type "`tag`" with a text attribute of `group(3)` + of a `pattern`. `tag` should be a string of a HTML element (i.e.: 'em'). + +* `SubstituteTagPattern(pattern, tag)`: + + Returns an element of type "`tag`" with no children or text (i.e.: 'br'). + +There may be other Pattern classes in the Markdown source that you could extend +or use as well. Read through the source and see if there is anything you can +use. You might even get a few ideas for different approaches to your specific +situation. + +<h3 id="postprocessors">Postprocessors</h3> + +Postprocessors manipulate a document after it has passed through the Markdown +core. This is were stored text gets added back in such as a list of footnotes, +a table of contents or raw html. + +There are two types of postprocessors: [ElementTree Postprocessors][] and +[TextPostprocessors][]. + +<h4 id="etpostprocessors">ElementTree Postprocessors</h4> + +An ElementTree Postprocessor should inherit from `markdown.Postprocessor`, +over-ride the `run` method which takes one argument `root` and return either +that root element or a modified root element. + +A pseudo example: + + class MyPostprocessor(markdown.Postprocessor): + def run(self, root): + #do stufff + return my_modified_root + +For specifics on manipulating the ElementTree, see +[Working with the ElementTree][] below. + +<h4 id="textpostprocessors">TextPostprocessors</h4> + +A TextPostprocessor should inherit from `markdown.TextPostprocessor` and +over-ride the `run` method which takes one argument `text` and returns a +Unicode string. + +TextPostprocessors are run after the ElementTree has been serialized back into +Unicode text. For example, this may be an appropriate place to add a table of +contents to a document: + + class TocTextPostprocessor(markdown.TextPostprocessor): + def run(self, text): + return MYMARKERRE.sub(MyToc, text) + +<h3 id="working_with_et">Working with the ElementTree</h3> + +As mentioned, the Markdown parser converts a source document to an +[ElementTree][] object before serializing that back to Unicode text. +Markdown has provided some helpers to ease that manipulation within the context +of the Markdown module. + +First, to get access to the ElementTree module import ElementTree from +``markdown`` rather than importing it directly. This will ensure you are using +the same version of ElementTree as markdown. The module is named ``etree`` +within Markdown. + + from markdown import etree + +``markdown.etree`` tries to import ElementTree from any known location, first +as a standard library module (from ``xml.etree`` in Python 2.5), then as a third +party package (``Elementree``). In each instance, ``cElementTree`` is tried +first, then ``ElementTree`` if the faster C implementation is not available on +your system. + +Sometimes you may want text inserted into an element to be parsed by +[InlinePatterns][]. In such a situation, simply insert the text into an +`inline` tag and the text will be automatically run through the InlinePatterns. + +Here's a basic example which creates an HTML table (note that the contents of +the second cell (``td2``) will be run through InlinePatterns latter): + + table = etree.Element("table") + table.set("cellpadding", "2") # Set cellpadding to 2 + tr = etree.SubElement(table, "tr") # Add child tr to table + td1 = etree.SubElement(tr, "td") # Add child td1 to tr + td1.text = "Cell content" # Add plain text content to td1 element + td2 = etree.SubElement(tr, "td") # Add second td to tr + inline = etree.SubElement(td2, "inline") # Add an inline element to td2 + inline.text = "Some *text* with **inline** formatting." # Add markup text + table.tail = "Text after table" # Added text after table Element + +You can also manipulate an existing tree. Consider the following example which +adds a ``class`` attribute to all ``a`` elements: + + def set_link_class(self, element): + for child in element: + if child.tag == "a": + child.set("class", "myclass") #set the class attribute + set_link_class(child) # run recursively on children + +For more information about working with ElementTree see the ElementTree +[Documentation](http://effbot.org/zone/element-index.htm) +([Python Docs](http://docs.python.org/lib/module-xml.etree.ElementTree.html)). + +<h3 id="integrating_into_markdown">Integrating Your Code Into Markdown + +Once you have the various pieces of your extension built, you need to tell +Markdown about them and ensure that they are run in the proper sequence. +Markdown accepts a `Extension` instance for each extension. Therefore, you +will need to define a class that extends `markdown.Extension` and over-rides +the `extendMarkdown` method. Within this class you will manage configuration +options for your extension and attach the various processors and patterns to +the Markdown instance. + +It is important to note that the order of the various processors and patterns +matters. For example, if we replace `http://...` links with <a> elements, and +*then* try to deal with inline html, we will end up with a mess. Therefore, +the various types of processors and patterns are stored within an instance of +the Markdown class within lists. Your `Extension` class will need to manipulate +those lists appropriately. You may insert instances of your processors and +patterns into the appropriate location in a list, remove a built-in instances, +or replace a built-in instance with your own. + +<h4 id="extendmarkdown">`extendMarkdown`</h4> + +The `extendMarkdown` method of a `markdown.Extension` class accepts two +arguments: + +* `md`: + + A pointer to the instance of the Markdown class. You should use this to + access the lists of processors and patterns. They are found under the + following attributes: + + * `md.textPreprocessors` + * `md.preprocessors` + * `md.inlinePatterns` + * `md.postpreprocessors` + * `md.textPostprocessors` + + Some other things you may want to access in the markdown instance are: + + * `md.inlineStash` + * `md.htmlStash` + * `md.registerExtension()` + +* `md_globals` + + Contains all the various global variables within the markdown module. + +Of course, with access to those items, theoretically you have the option to +changing anything through various [monkey_patching][] techniques. In fact, this +is how both the [[HeaderId]] and [[CodeHilite]] extensions work. However, you +should be aware that the various undocumented or private parts of markdown may +change without notice and your monkey_patches may break with a new release. +Therefore, what you really should be doing is inserting processors and patterns +into the markdown pipeline. Consider yourself warned. + +[monkey_patching]: http://en.wikipedia.org/wiki/Monkey_patch + +<h4 id="registerextension">registerExtension</h4> + +Some extensions may need to have their state reset between multiple runs of the +Markdown class. For example, consider the following use of the [[Footnotes]] +extension: + + md = markdown.Markdown(extensions=['footnotes']) + html1 = md.convert(text_with_footnote) + md.reset() + html2 = md.convert(text_without_footnote) + +Without calling ``reset``, the footnote definitions from the first document will +be inserted into the second document as they are still stored within the class +instance. Therefore the ``Extension`` class needs to define a ``reset`` method +that will reset the state of the extension (i.e.: ``self.footnotes = {}``). +However, as many extensions do not have a need for ``reset``, ``reset`` is only +called on extensions that are registered. + +To register an extension, call ``md.registerExtension`` from within your +``extendMarkdown`` method: + + + def extendMarkdown(self, md, md_globals): + md.registerExtension(self) + # insert processors and patterns here + +Then, each time ``reset`` is called on the Markdown instance, the ``reset`` +method of each registered extension will be called as well. You should also +note that ``reset`` will be called on each registered extension after it is +initialized the first time. Keep that in mind when over-riding the extension's +``reset`` method. + +<h4 id="configsettings">Config Settings</h4> + +If an extension uses any parameters that the user may want to change, +those parameters should be stored in `self.config` of your `markdown.Extension` +class in the following format: + + self.config = {parameter_1_name : [value1, description1], + parameter_2_name : [value2, description2] } + +When stored this way the config parameters can be over-ridden from the +command line or at the time Markdown is initiated: + + markdown.py -x myextension(SOME_PARAM=2) inputfile.txt > output.txt + +Note that parameters should always be assumed to be set to string +values, and should be converted at run time. For example: + + i = int(self.getConfig("SOME_PARAM")) + +<h4 id="makeextension">`makeExtension`</h4> + +Each extension should ideally be placed in its own module starting +with the ``mdx_`` prefix (e.g. ``mdx_footnotes.py``). The module must +provide a module-level function called ``makeExtension`` that takes +an optional parameter consisting of a dictionary of configuration over-rides +and returns an instance of the extension. An example from the footnote +extension: + + def makeExtension(configs=None) : + return FootnoteExtension(configs=configs) + +By following the above example, when Markdown is passed the name of your +extension as a string (i.e.: ``'footnotes'``), it will automatically import +the module and call the ``makeExtension`` function initiating your extension. + +You may have noted that the extensions packaged with Python-Markdown do not +use the ``mdx_`` prefix in their module names. This is because they are all +part of the ``markdown_extensions`` package. Markdown will first try to import +from ``markdown_extensions.extname`` and upon failure, ``mdx_extname``. If both +fail, Markdown will continue without the extension. + +However, Markdown will also accept an already existing instance of an extension. +For example: + + import markdown + import myextension + configs = {...} + myext = myextension.MyExtension(configs=configs) + md = markdown.Markdown(extensions=[myext]) + +This is useful if you need to implement a large number of extensions with more +than one residing in a module. + +[Preprocessors]: #preprocessors +[TextPreprocessors]: #textpreprocessors +[Line Preprocessors]: #linepreprocessors +[InlinePatterns]: #inlinepatterns +[Postprocessors]: #postprocessors +[ElementTree Postprocessors]: #etpostprocessors +[TextProstprocessors]: #textpostprocessors +[Working with the ElementTree]: #working_with_et +[Integrating your code into Markdown]: #integrating_into_markdown +[extendMarkdown]: #extendmarkdown +[registerExtension]: #registerextension +[Config Settings]: #configsettings +[makeExtension]: #makeextension +[ElementTree]: http://effbot.org/zone/element-index.htm |