Custom action on code fences but keep the normal behavior

[oprypin]

Hi. I want to validate code blocks in my MkDocs site (e.g. by giving them to an external process). The validation would basically have the ability to abort building the site. A further ability that I'd like to have is to modify the code snippet before passing it onwards.

Other than that, there should be no changes to what code fences do, and normal syntax highlighting should continue to work.
Ideally I'd implement a reusable plugin, not a one-off.

---

So I started looking, how do I get the content (and class) of code fences?

• I don't want to reimplement the parsing already done by SuperFences.
• A treeprocessor (after superfences processing) can't work, because it just sees the htmlStash placeholders.
• Looking at htmlStash itself is weird because it's already HTML. Also it has already discarded classes. It could kinda work, though.
• Should I monkeypatch/wrap the highlight function (through treeprocessors["fenced_code_block"])? Really nasty but happens to satisfy all my requirements.
  

  pymdown-extensions/pymdownx/superfences.py

  Line 739 in 390b15d

  def highlight(self, src="", language="", options=None, md=None, **kwargs):

---

Oh wait, "custom fences" is the right feature!

• Should I do my check inside the validator and just always return False? It would probably work OK but
  • I have no ability to modify the code snippet
  • Requires specifying the "custom fences" config manually (but ideally I want a fully code-based extension).
    • Or is it feasible to somehow reach this function from my code?

      pymdown-extensions/pymdownx/superfences.py

      Line 242 in 390b15d

      def extend_super_fences(self, name, formatter, validator):

      Would I need to look through registeredExtensions or something?
• Should I monkeypatch/wrap the specific item SuperFencesCodeExtension.superfences[0]?
  

  pymdown-extensions/pymdownx/superfences.py

  Line 316 in 390b15d

  self.superfences[0]["formatter"] = fenced.highlight

• Should I extract the existing SuperFencesBlockPreprocessor just to get to its highlight method and use it as the formatter?
• Should I instantiate another SuperFencesBlockPreprocessor with manual config and use its highlight method?

---

Other options?

Basically my big issue with the supported approach is that there's no good way to still use the default highlight behavior if I'm overriding it with my "custom fence" action.

Or, well, going back to the overall problem, I basically just want a hook to be called on every fenced code block, without abandoning the highlighting behavior.

[facelessuser]

Yeah, you can use custom fences and call into the default formatter. I basically do that in this project: https://github.com/facelessuser/coloraide/blob/master/tools/colors.py#L210.

Basically, the above code is allowing me to parse raw Python code and get it's results, then I format it as a pycon input/output, send it through the highlighter and also about color previews:

```color
Color("rebeccapurple").interpolate(
    "lch(85% 100 85)",
    space='lch'
)
```

Generally, it is assumed that if someone is overriding this, they don't want to do pygments parsing, or they are going to implement their own; otherwise, they would normally just use the default highlighter. But yeah, I ran into a situation where I wanted to do much more, but still, leverage the highlighter. Custom fence usage is one of those things that has kind of evolved over time. It's one of those things, that given time, I push a bit further.

[oprypin]

Thanks!
So then, the expression md.preprocessors['fenced_code_block'].extension.superfences[0]['formatter'] has some level of backwards compatibility guarantee 😄

[facelessuser]

Yeah, some at least 🙂.

[oprypin]

Here's what I did with this:

MkDocs plugin

• Injecting config into pymdownx.superfences
• Fallback to [0]['formatter']

[facelessuser]

Very cool. Glad you were able to build something useful with it.