I was looking through the uml-diagram-example, but I can't figure out how to implement custom formatter instead of using one of the default ones.

In the example, processing is done by mermaid library on the client side. Instead, I want to process content of a superfenced block on backend during mkdocs build.

That's what I have in my config file:

markdown_extensions:
  ...
  - pymdownx.superfences
      custom_fences:
        - name: graphviz
          class: graphviz
          format: !!python/name:<custom formatter function>

In the docs there is an example format function:

def custom_format(source, language, class_name, options, md):
    """Custom format."""

    return '<div class_name="%s %s", data-option="%s">%s</div>' % (language, class_name, options['opt'], html_escape(source))

Where should I put it?

Should I now create a new python module and install it or it's possible to access python source code from local file?