dev-guide.md

Developer's Guide

This document describes usage of mistletoe and its API from the developer's point of view.

Understanding the AST

When a markdown document gets parsed by mistletoe, the result is represented as an "abstract syntax tree" (AST), stored in an instance of Document. This object contains a hierarchy of all the various tokens which were recognized during the parsing process.

In order to see what exactly gets parsed, one can simply use the ASTRenderer on a given markdown input, for example:

mistletoe text.md --renderer mistletoe.ast_renderer.ASTRenderer

Say that the input file contains for example:

# Heading 1

text

# Heading 2

[link](https://www.example.com)

Then we will get this JSON output from the AST renderer:

{
  "type": "Document",
  "footnotes": {},
  "children": [
    {
      "type": "Heading",
      "level": 1,
      "children": [
        {
          "type": "RawText",
          "content": "Heading 1"
        }
      ]
    },
    {
      "type": "Paragraph",
      "children": [
        {
          "type": "RawText",
          "content": "text"
        }
      ]
    },
    {
      "type": "Heading",
      "level": 1,
      "children": [
        {
          "type": "RawText",
          "content": "Heading 2"
        }
      ]
    },
    {
      "type": "Paragraph",
      "children": [
        {
          "type": "Link",
          "target": "https://www.example.com",
          "title": "",
          "children": [
            {
              "type": "RawText",
              "content": "link"
            }
          ]
        }
      ]
    }
  ]
}

When passing this tree to a renderer, it is recursively traversed and methods corresponding to individual token types get called on the renderer in order to create the output in the desired format.

Creating a custom renderer

Here's an example of how to add GitHub-style wiki links to the parsing process, and provide a renderer for this new token.

A new token

GitHub wiki links are span-level tokens, meaning that they reside inline, and don't really look like chunky paragraphs. To write a new span-level token, all we need to do is make a subclass of SpanToken:

from mistletoe.span_token import SpanToken

class GithubWiki(SpanToken):
    pass

mistletoe uses regular expressions to search for span-level tokens in the parsing process. As a refresher, GitHub wiki looks something like this: [[alternative text | target]]. We define a class variable, pattern, that stores the compiled regex:

class GithubWiki(SpanToken):
    pattern = re.compile(r"\[\[ *(.+?) *\| *(.+?) *\]\]")
    def __init__(self, match):
        pass

The regex will be picked up by SpanToken.find, which is used by the tokenizer to find all tokens of its kind in the document. If regexes are too limited for your use case, consider overriding the find method; it should return a list of all token occurrences.

Three other class variables are available for our custom token class, and their default values are shown below:

class SpanToken:
    parse_group = 1
    parse_inner = True
    precedence = 5

Note that alternative text can also contain other span-level tokens. For example, [[*alt*|link]] is a GitHub link with an Emphasis token as its child. To parse child tokens, parse_inner should be set to True (the default value in this case), and parse_group should correspond to the match group in which child tokens might occur (also the default value, 1, in this case).

Once these two class variables are set correctly, GithubWiki.children attribute will automatically be set to the list of child tokens. Note that there is no need to manually set this attribute, unlike previous versions of mistletoe.

Lastly, the SpanToken constructors take a regex match object as its argument. We can simply store off the target attribute from match_obj.group(2).

from mistletoe.span_token import SpanToken

class GithubWiki(SpanToken):
    pattern = re.compile(r"\[\[ *(.+?) *\| *(.+?) *\]\]")
    def __init__(self, match_obj):
        self.target = match_obj.group(2)

There you go: a new token in 5 lines of code.

Side note about precedence

Normally there is no need to override the precedence value of a custom token. The default value is the same as InlineCode, AutoLink and HTMLSpan, which means that whichever token comes first will be parsed. In our case:

`code with [[ text` | link ]]

... will be parsed as:

<code>code with [[ text</code> | link ]]

If we set GithubWiki.precedence = 6, we have:

`code with <a href="link">text`</a>

A new renderer

Adding a custom token to the parsing process usually involves a lot of nasty implementation details. Fortunately, mistletoe takes care of most of them for you. Simply passing your custom token class to super().__init__() does the trick:

from mistletoe.html_renderer import HTMLRenderer

class GithubWikiRenderer(HTMLRenderer):
    def __init__(self):
        super().__init__(GithubWiki)

We then only need to tell mistletoe how to render our new token:

def render_github_wiki(self, token):
    template = '<a href="{target}">{inner}</a>'
    target = token.target
    inner = self.render_inner(token)
    return template.format(target=target, inner=inner)

Cleaning up, we have our new renderer class:

from mistletoe.html_renderer import HTMLRenderer, escape_url

class GithubWikiRenderer(HTMLRenderer):
    def __init__(self):
        super().__init__(GithubWiki)

    def render_github_wiki(self, token):
        template = '<a href="{target}">{inner}</a>'
        target = escape_url(token.target)
        inner = self.render_inner(token)
        return template.format(target=target, inner=inner)

Take it for a spin?

It is preferred that all mistletoe's renderers be used as context managers. This is to ensure that your custom tokens are cleaned up properly, so that you can parse other Markdown documents with different token types in the same program.

from mistletoe import Document
from contrib.github_wiki import GithubWikiRenderer

with open('foo.md', 'r') as fin:
    with GithubWikiRenderer() as renderer:
        rendered = renderer.render(Document(fin))

For more info, take a look at the base_renderer module in mistletoe. The docstrings might give you a more granular idea of customizing mistletoe to your needs.