Writing Extensions for Python-Markdown

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. If you really want to dive in, there are also Blockprocessors which are part of the core BlockParser.

As the parser builds an ElementTree object which is later rendered as Unicode text, there are also some helpers provided to ease manipulation of the tree. Each part of the API is discussed in its respective section below. Additionally, reading the source of some Available Extensions may be helpful. For example, the Footnotes extension uses most of the features documented here.

Preprocessors

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.

Preprocessors should inherit from markdown.preprocessors.Preprocessor and implement a run method with one argument lines. The run method of each 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:

from markdown.preprocessors import Preprocessor

class MyPreprocessor(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

Inline Patterns

Legacy

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.inlinepatterns.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.

Also, Inline Patterns can define the property ANCESTOR_EXCLUDES with either a list or tuple of undesirable ancestors. The pattern should not match if it would cause the content to be a descendant of one of the defined tag names.

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 and set the re.DOTALL and re.UNICODE flags. 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:

from markdown.inlinepatterns import Pattern
from markdown.util import etree

class EmphasisPattern(Pattern):
    def handleMatch(self, m):
        el = etree.Element('em')
        el.text = m.group(2)
        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 SimpleTagPattern listed below. Feel free to use or extend any of the Pattern classes found at markdown.inlinepatterns.

Future

While users can still create plugins with the existing markdown.inlinepatterns.Pattern, a new, more flexible inline processor has been added which users are encouraged to migrate to. The new inline processor is found at markdown.inlinepatterns.InlineProcessor.

The new processor is very similar to legacy with two major distinctions.

  1. Patterns no longer need to match the entire block, so patterns no longer start with r'^(.*?)' and end with r'(.*?)!'. This was a huge performance sink and this requirement has been removed. The returned match object will only contain what is explicitly matched in the pattern, and extension pattern groups now start with m.group(1).

  2. The handleMatch method now takes an additional input called data, which is the entire block under analysis, not just what is matched with the specified pattern. The method also returns the element and the index boundaries relative to data that the return element is replacing (usually m.start(0) and m.end(0)). If the boundaries are returned as None, it is assumed that the match did not take place, and nothing will be altered in data.

If all you need is the same functionality as the legacy processor, you can do as shown below. Most of the time, simple regular expression processing is all you’ll need.

from markdown.inlinepatterns import InlineProcessor
from markdown.util import etree

# an oversimplified regex
MYPATTERN = r'\*([^*]+)\*'

class EmphasisPattern(InlineProcessor):
    def handleMatch(self, m, data):
        el = etree.Element('em')
        el.text = m.group(1)
        return el, m.start(0), m.end(0)

# pass in pattern and create instance
emphasis = EmphasisPattern(MYPATTERN)

But, the new processor allows you handle much more complex patterns that are too much for Python’s Re to handle. For instance, to handle nested brackets in link patterns, the built-in link inline processor uses the following pattern to find where a link might start:

LINK_RE = NOIMG + r'\['
link = LinkInlineProcessor(LINK_RE, md_instance)

It then uses programmed logic to actually walk the string (data), starting at where the match started (m.start(0)). If for whatever reason, the text does not appear to be a link, it returns None for the start and end boundary in order to communicate to the parser that no match was found.

    # Just a snippet of the link's handleMatch
    # method to illustrate new logic
    def handleMatch(self, m, data):
        text, index, handled = self.getText(data, m.end(0))

        if not handled:
            return None, None, None

        href, title, index, handled = self.getLink(data, index)
        if not handled:
            return None, None, None

        el = util.etree.Element("a")
        el.text = text

        el.set("href", href)

        if title is not None:
            el.set("title", title)

        return el, m.start(0), index

Generic Pattern Classes

Some example processors that are available.

  • SimpleTextInlineProcessor(pattern):

    Returns simple text of group(2) of a pattern and the start and end position of the match.

  • SimpleTagInlineProcessor(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’). It also returns the start and end position of the match.

  • SubstituteTagInlineProcessor(pattern, tag):

    Returns an element of type “tag” with no children or text (i.e.: br) and the start and end position of the match.

A very small number of the basic legacy processors are still available to prevent breakage of 3rd party extensions during the transition period to the new processors. Three of the available processors are listed below.

  • 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.

Treeprocessors

Treeprocessors manipulate an ElementTree object after it has passed through the core BlockParser. This is where additional manipulation of the tree takes place. Additionally, the InlineProcessor is a Treeprocessor which steps through the tree and runs the Inline Patterns on the text of each Element in the tree.

A Treeprocessor should inherit from markdown.treeprocessors.Treeprocessor, over-ride the run method which takes one argument root (an ElementTree object) and either modifies that root element and returns None or returns a new ElementTree object.

A pseudo example:

from markdown.treeprocessors import Treeprocessor

class MyTreeprocessor(Treeprocessor):
    def run(self, root):
        root.text = 'modified content'

Note that Python class methods return None by default when no return statement is defined. Additionally all Python variables refer to objects by reference. Therefore, the above run method modifies the root element in place and returns None. The changes made to the root element and its children are retained.

Some may be inclined to return the modified root element. While that would work, it would cause a copy of the entire ElementTree to be generated each time the Treeprocessor is run. Therefore, it is generally expected that the run method would only return None or a new ElementTree object.

For specifics on manipulating the ElementTree, see Working with the ElementTree below.

Postprocessors

Postprocessors manipulate the document after the ElementTree has been serialized into a string. Postprocessors should be used to work with the text just before output.

A Postprocessor should inherit from markdown.postprocessors.Postprocessor and over-ride the run method which takes one argument text and returns a Unicode string.

Postprocessors 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:

from markdown.postprocessors import Postprocessor

class TocPostprocessor(Postprocessor):
    def run(self, text):
        return MYMARKERRE.sub(MyToc, text)

BlockParser

Sometimes, Preprocessors, Treeprocessors, Postprocessors, and Inline Patterns are not going to do what you need. Perhaps you want a new type of block type that needs to be integrated into the core parsing. In such a situation, you can add/change/remove functionality of the core BlockParser. The BlockParser is composed of a number of Blockprocessors. The BlockParser steps through each block of text (split by blank lines) and passes each block to the appropriate Blockprocessor. That Blockprocessor parses the block and adds it to the ElementTree. The Definition Lists extension would be a good example of an extension that adds/modifies Blockprocessors.

A Blockprocessor should inherit from markdown.blockprocessors.BlockProcessor and implement both the test and run methods.

The test method is used by BlockParser to identify the type of block. Therefore the test method must return a Boolean value. If the test returns True, then the BlockParser will call that Blockprocessor’s run method. If it returns False, the BlockParser will move on to the next Blockprocessor.

The test method takes two arguments:

  • parent: The parent ElementTree Element of the block. This can be useful as the block may need to be treated differently if it is inside a list, for example.

  • block: A string of the current block of text. The test may be a simple string method (such as block.startswith(some_text)) or a complex regular expression.

The run method takes two arguments:

  • parent: A pointer to the parent ElementTree Element of the block. The run method will most likely attach additional nodes to this parent. Note that nothing is returned by the method. The ElementTree object is altered in place.

  • blocks: A list of all remaining blocks of the document. Your run method must remove (pop) the first block from the list (which it altered in place - not returned) and parse that block. You may find that a block of text legitimately contains multiple block types. Therefore, after processing the first type, your processor can insert the remaining text into the beginning of the blocks list for future parsing.

Please be aware that a single block can span multiple text blocks. For example, The official Markdown syntax rules state that a blank line does not end a Code Block. If the next block of text is also indented, then it is part of the previous block. Therefore, the BlockParser was specifically designed to address these types of situations. If you notice the CodeBlockProcessor, in the core, you will note that it checks the last child of the parent. If the last child is a code block (<pre><code>...</code></pre>), then it appends that block to the previous code block rather than creating a new code block.

Each Blockprocessor has the following utility methods available:

  • lastChild(parent):

    Returns the last child of the given ElementTree Element or None if it had no children.

  • detab(text):

    Removes one level of indent (four spaces by default) from the front of each line of the given text string.

  • looseDetab(text, level):

    Removes “level” levels of indent (defaults to 1) from the front of each line of the given text string. However, this methods allows secondary lines to not be indented as does some parts of the Markdown syntax.

Each Blockprocessor also has a pointer to the containing BlockParser instance at self.parser, which can be used to check or alter the state of the parser. The BlockParser tracks it’s state in a stack at parser.state. The state stack is an instance of the State class.

State is a subclass of list and has the additional methods:

  • set(state):

    Set a new state to string state. The new state is appended to the end of the stack.

  • reset():

    Step back one step in the stack. The last state at the end is removed from the stack.

  • isstate(state):

    Test that the top (current) level of the stack is of the given string state.

Note that to ensure that the state stack does not become corrupted, each time a state is set for a block, that state must be reset when the parser finishes parsing that block.

An instance of the BlockParser is found at Markdown.parser. BlockParser has the following methods:

  • parseDocument(lines):

    Given a list of lines, an ElementTree object is returned. This should be passed an entire document and is the only method the Markdown class calls directly.

  • parseChunk(parent, text):

    Parses a chunk of markdown text composed of multiple blocks and attaches those blocks to the parent Element. The parent is altered in place and nothing is returned. Extensions would most likely use this method for block parsing.

  • parseBlocks(parent, blocks):

    Parses a list of blocks of text and attaches those blocks to the parent Element. The parent is altered in place and nothing is returned. This method will generally only be used internally to recursively parse nested blocks of text.

While it is not recommended, an extension could subclass or completely replace the BlockParser. The new class would have to provide the same public API. However, be aware that other extensions may expect the core parser provided and will not work with such a drastically different parser.

Working with the ElementTree

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 found at markdown.util.etree within Markdown.

from markdown.util import etree

markdown.util.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 (ElementTree). 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 Inline Patterns. In such a situation, simply insert the text as you normally would and the text will be automatically run through the Inline Patterns. However, if you do not want some text to be parsed by Inline Patterns, then insert the text as an AtomicString.

from markdown.util import AtomicString
some_element.text = AtomicString(some_text)

Here’s a basic example which creates an HTML table (note that the contents of the second cell (td2) will be run through Inline Patterns 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 = markdown.util.AtomicString("Cell content") # Add plain text content
td2 = etree.SubElement(tr, "td")                   # Add second td to tr
td2.text = "*text* with **inline** formatting."    # Add markup text
table.tail = "Text after table"                    # Add text after table

You can also manipulate an existing tree. Consider the following example which adds a class attribute to <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 (Python Docs).

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 an Extension instance for each extension. Therefore, you will need to define a class that extends markdown.extensions.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 in a Registry. Your Extension class will need to manipulate those registries appropriately. You may register instances of your processors and patterns with an appropriate priority, deregister built-in instances, or replace a built-in instance with your own.

extendMarkdown

The extendMarkdown method of a markdown.extensions.Extension class accepts one argument:

  • md:

    A pointer to the instance of the Markdown class. You should use this to access the Registries of processors and patterns. They are found under the following attributes:

    • md.preprocessors
    • md.inlinePatterns
    • md.parser.blockprocessors
    • md.treeprocessors
    • md.postprocessors

    Some other things you may want to access in the markdown instance are:

    • md.htmlStash
    • md.output_formats
    • md.set_output_format()
    • md.output_format
    • md.serializer
    • md.registerExtension()
    • md.tab_length
    • md.block_level_elements
    • md.isBlockLevel()

Warning

With access to the above items, theoretically you have the option to change anything through various monkey_patching techniques. However, you should be aware that the various undocumented 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!

A simple example:

from markdown.extensions import Extension

class MyExtension(Extension):
    def extendMarkdown(self, md):
        # Register instance of 'mypattern' with a priority of 175
        md.inlinePatterns.register(MyPattern(md), 'mypattern', 175)

Registry

The markdown.util.Registry class is a priority sorted registry which Markdown uses internally to determine the processing order of its various processors and patterns.

A Registry instance provides two public methods to alter the data of the registry: register and deregister. Use register to add items and deregister to remove items. See each method for specifics.

When registering an item, a “name” and a “priority” must be provided. All items are automatically sorted by “priority” from highest to lowest. The “name” is used to remove (deregister) and get items.

A Registry instance is like a list (which maintains order) when reading data. You may iterate over the items, get an item and get a count (length) of all items. You may also check that the registry contains an item.

When getting an item you may use either the index of the item or the string-based “name”. For example:

registry = Registry()
registry.register(SomeItem(), 'itemname', 20)
# Get the item by index
item = registry[0]
# Get the item by name
item = registry['itemname']

When checking that the registry contains an item, you may use either the string-based “name”, or a reference to the actual item. For example:

someitem = SomeItem()
registry.register(someitem, 'itemname', 20)
# Contains the name
assert 'itemname' in registry
# Contains the item instance
assert someitem in registry

markdown.util.Registry has the following methods:

Registry.register(self, item, name, priority)

Add an item to the registry with the given name and priority.

Parameters:

  • item: The item being registered.
  • name: A string used to reference the item.
  • priority: An integer or float used to sort against all items.

If an item is registered with a “name” which already exists, the existing item is replaced with the new item. Tread carefully as the old item is lost with no way to recover it. The new item will be sorted according to its priority and will not retain the position of the old item.

Registry.deregister(self, name, strict=True)

Remove an item from the registry.

Set strict=False to fail silently.

Registry.get_index_for_name(self, name)

Return the index of the given name.

registerExtension

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.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.

Configuration Settings

If an extension uses any parameters that the user may want to change, those parameters should be stored in self.config of your markdown.extensions.Extension class in the following format:

class MyExtension(markdown.extensions.Extension):
    def __init__(self, **kwargs):
        self.config = {'option1' : ['value1', 'description1'],
                       'option2' : ['value2', 'description2'] }
        super(MyExtension, self).__init__(**kwargs)

When implemented this way the configuration parameters can be over-ridden at run time (thus the call to super). For example:

markdown.Markdown(extensions=[MyExtension(option1='other value'])

Note that if a keyword is passed in that is not already defined in self.config, then a KeyError is raised.

The markdown.extensions.Extension class and its subclasses have the following methods available to assist in working with configuration settings:

  • getConfig(key [, default]):

    Returns the stored value for the given key or default if the key does not exist. If not set, default returns an empty string.

  • getConfigs():

    Returns a dict of all key/value pairs.

  • getConfigInfo():

    Returns all configuration descriptions as a list of tuples.

  • setConfig(key, value):

    Sets a configuration setting for key with the given value. If key is unknown, a KeyError is raised. If the previous value of key was a Boolean value, then value is converted to a Boolean value. If the previous value of key is None, then value is converted to a Boolean value except when it is None. No conversion takes place when the previous value of key is a string.

  • setConfigs(items):

    Sets multiple configuration settings given a dict of key/value pairs.

Naming an Extension

As noted in the library reference an instance of an extension can be passed directly to Markdown. In fact, this is the preferred way to use third-party extensions.

For example:

import markdown
from path.to.module import MyExtension
md = markdown.Markdown(extensions=[MyExtension(option='value')])

However, Markdown also accepts “named” third party extensions for those occasions when it is impractical to import an extension directly (from the command line or from within templates). A “name” can either be a registered entry point or a string using Python’s dot notation.

Entry Point

Entry points are defined in a Python package’s setup.py script. The script must use setuptools to support entry points. Python-Markdown extensions must be assigned to the markdown.extensions group. An entry point definition might look like this:

from setuptools import setup

setup(
    # ...
    entry_points={
        'markdown.extensions': ['myextension = path.to.module:MyExtension']
    }
)

After a user installs your extension using the above script, they could then call the extension using the myextension string name like this:

markdown.markdown(text, extensions=['myextension'])

Note that if two or more entry points within the same group are assigned the same name, Python-Markdown will only ever use the first one found and ignore all others. Therefore, be sure to give your extension a unique name.

For more information on writing setup.py scripts, see the Python documentation on Packaging and Distributing Projects.

Dot Notation

If an extension does not have a registered entry point, Python’s dot notation may be used instead. The extension must be installed as a Python module on your PYTHONPATH. Generally, a class should be specified in the name. The class must be at the end of the name and be separated by a colon from the module.

Therefore, if you were to import the class like this:

from path.to.module import MyExtension

Then the extension can be loaded as follows:

markdown.markdown(text, extensions=['path.to.module:MyExtension'])

You do not need to do anything special to support this feature. As long as your extension class is able to be imported, a user can include it with the above syntax.

The above two methods are especially useful if you need to implement a large number of extensions with more than one residing in a module. However, if you do not want to require that your users include the class name in their string, you must define only one extension per module and that module must contain a module-level function called makeExtension that accepts **kwargs and returns an extension instance.

For example:

class MyExtension(markdown.extensions.Extension)
    # Define extension here...

def makeExtension(**kwargs):
    return MyExtension(**kwargs)

When Markdown is passed the “name” of your extension as a dot notation string that does not include a class (for example path.to.module), it will import the module and call the makeExtension function to initiate your extension.