mdformat-front-matters

An mdformat plugin for normalizing YAML, TOML, and JSON front matter in CommonMark documents.

Note

mdformat-frontmatter has additional duplicate key detection, but did not support mdformat v1 (butler54/mdformat-frontmatter #37) nor TOML and JSON at the time (butler54/mdformat-frontmatter#22 (comment))

Along with the 's', the extra dash is intentional to try to prevent typo errors.

Features

• Multi-format support: Handles YAML (---), TOML (+++), and JSON ({...}) front matter
• Automatic normalization: Formats front matter consistently (preserves key order by default, standardized indentation)
• Configurable sorting: Option to sort keys alphabetically with --sort-front-matter
• Error resilient: Preserves original content if parsing fails. Will error only if strict mode is set
• Zero configuration: Works out of the box with mdformat

Examples

YAML Front Matter:

---
title: My Document
date: 2024-01-01
tags:
  - example
  - demo
---

# Content

With --sort-front-matter, becomes:

---
date: 2024-01-01
tags:
  - example
  - demo
title: My Document
---

# Content

TOML Front Matter:

+++
title = "My Document"
date = 2024-01-01
tags = ["example", "demo"]
+++

# Content

JSON Front Matter:

{
    "title": "My Document",
    "date": "2024-01-01",
    "tags": ["example", "demo"]
}

# Content

mdformat Usage

Add this package wherever you use mdformat and the plugin will be auto-recognized. No additional configuration necessary. See additional information on mdformat plugins here

pre-commit / prek

repos:
  - repo: https://github.com/executablebooks/mdformat
    rev: 1.0.0
    hooks:
      - id: mdformat
        additional_dependencies:
          - mdformat-front-matters

uvx

uvx --with mdformat-front-matters mdformat

Or with pipx:

pipx install mdformat
pipx inject mdformat mdformat-front-matters

Configuration Options

Key Sorting

By default, front matter keys preserve their original order. To sort keys alphabetically for consistency, use the --sort-front-matter flag.

# Default behavior - preserves original key order
mdformat document.md

# Sort keys alphabetically
mdformat document.md --sort-front-matter

Strict Mode

Enable strict mode to fail on invalid front matter instead of preserving it. Useful for CI/CD pipelines.

mdformat document.md --strict-front-matter

In strict mode:

• Invalid front matter raises an error
• Front matter without valid key-value pairs raises an error
• Ensures your documents have correctly formatted metadata

Example usage in pre-commit:

repos:
  - repo: https://github.com/executablebooks/mdformat
    rev: 0.7.19
    hooks:
      - id: mdformat
        args: [--strict-front-matter]
        additional_dependencies:
          - mdformat-front-matters

HTML Rendering

To hide Front Matter from generated HTML output, front_matters_plugin can be imported from mdit_plugins. For more guidance on MarkdownIt, see the docs: https://markdown-it-py.readthedocs.io/en/latest/using.html#the-parser

from markdown_it import MarkdownIt

from mdformat_front_matters.mdit_plugins import front_matters_plugin

md = MarkdownIt()
md.use(front_matters_plugin)

text = """
+++
title = "Example"
draft = false
+++
# Example
"""
md.render(text)
# <h1>Example</h1>

Contributing

See CONTRIBUTING.md